Praise be to Allah.
Seeing Allaah in reality (as opposed to in a dream) is not possible in this life, although no doubt it will happen after death. This is not because seeing Him is impossible in itself, but because Allaah has willed that it should not happen. This is for many reasons, some of which have been found by the scholars, and others which remain known only to Allaah. Among the reasons which we do understand is human weakness, which is apparent from the story of Moosa (upon whom be peace), which we will quote shortly. Another reason is that seeing Allaah is a great blessing and joy, indeed it is the greatest blessing, so it is withheld for the place of ultimate blessing and joy, namely Paradise. Another reason is that this world is a mixture of good believers and kuffaar, so this blessing is deferred until it will be given exclusively to the believers in Paradise. Withholding this blessing until the Hereafter also provides a strong motive to do good in this world, so that one may see Allaah and feel secure and content in nearness to Him in the Hereafter.
Evidence that it is not possible to see Allaah in this world
Allaah says, in the story of Moosa (peace be upon him) (interpretation of the meaning): And when Moosa came at the time and place appointed by Us, and his Lord spoke to him, he said, O my Lord! Show me (Yourself), that I may look upon You. Allaah said, You cannot see Me, but look upon the mountain, if its stands still in its place, then you shall see Me. So when his Lord appeared to the mountain, He made it collapse to dust, and Moosa fell down unconscious. Then when he recovered his senses he said, Glory be to You, I turn to You in repentance and I am the first of the believers. [al-Araaf 7:143]. The word lan (here translated as cannot in You cannot see Me) conveys the meaning of You will not see Me in this life, because there is evidence that man will see Allaah in the Hereafter, which means that this lan is not eternal in application. (The negative particle lan often carries a sense of eternal negation, that something will never happen, but this is not the case here. Translator).
Allaah says (interpretation of the meaning): No vision can grasp Him, but His Grasp is over all vision. He is the Most Subtle and Courteous, Well-Acquainted with all things. [al-Anaam 6:103]. The commentator al-Haafiz Ibn Katheer (may Allaah have mercy on him) said, in his tafseer of this aayah: Concerning the phrase No vision can grasp Him, several comments regarding it have been narrated from the imaams of the salaf, one of which is that it means, you will not be able to see Him in this world, even though you will see Him in the Hereafter. The reports that state this, narrated from the Messenger of Allaah (peace and blessings of Allaah be upon him), reach the level of mutawaatir and are reported through more than one sound isnaad in the books of Saheeh, Musnad and Sunan.
Allaah says (interpretation of the meaning): It is not given to any human being that Allaah should speak to him unless (it be) by inspiration, or from behind a veil, or (that) He sends a Messenger to reveal what He will by His Leave. Verily, He is Most High, Most Wise. [al-Shoora 42:51].
Abu Hurayrah said: Some people said, O Messenger of Allaah, will we see our Lord on the Day of Resurrection? He said, Do you have any doubt about seeing the sun on a cloudless day? They said, No, O Messenger of Allaah. He said, Do you have any doubt about seeing the full moon on a cloudless night? They said, No, O Messenger of Allaah. He said, You will see Him just as clearly on the Day of Resurrection(Reported by al-Bukhaari, 6088). It is clear that the idea that Allaah will not be seen in this life was well-entrenched in the minds of the Sahaabah. They were asking about the Hereafter, and this is what the Prophets answer also referred to.
Abu Dharr said: I asked the Messenger of Allaah (peace and blessings of Allaah be upon him), Have you seen your Lord? He said, (There is) light, how could I see Him? This light, which prevented him from seeing Him, is a veil of light, which is further explained in the hadeeth of Abu Moosa, who said, The Messenger of Allaah (peace and blessings of Allaah be upon him) stood up and told us five things: Allaah does not sleep, and it is not befitting that He should sleep. He lowers the scale and lifts it. The deeds in the night are taken up to Him before the deeds of the day, and the deeds of the day before the deeds of the night. His veil is the light. If He were to withdraw it (the veil), the splendour of His countenance would consume His creation as far as His sight reaches.(Reported by Imaam Ahmad and Muslim, 263).
Further clear evidence comes in the hadeeth which warns against the Dajjaal (antichrist): Written between his eyes is (the word) kaafir, which will be read by everyone who hates his works, or every believer will read it. Know that none of you will see your Lord until he dies.(Reported by Muslim, 5215).
So any claim that anyone but the Prophet (peace and blessings of Allaah be upon him) can see Allaah in reality is impossible, according to the consensus of the scholars. However, there is some disagreement among the scholars as to whether or not the Prophet (peace and blessings of Allaah be upon him) saw Allaah on the night of the Miraaj. The correct view is that he did not see Him physically, with his own two eyes, because when he was asked about this, he said, (There is) light, how could I see Him? According to another report, he said, I saw light. The Prophet (peace and blessings of Allaah be upon him) did not see Him, but he saw His veil of light.
The question of whether Allaah may be seen in a dream does not contradict the fact that Allaah cannot be seen in reality in this world, because all the texts quoted above speak about physical vision when one is awake, not the vision of the heart when one is asleep. Proof that the latter kind of vision is possible is to be seen in the hadeeth about the dispute of the chiefs on high (the angels). Ibn Abbaas (may Allaah be pleased with him) said, The Messenger of Allaah (peace and blessings of Allaah be upon him) said, My Lord came to me tonight in the most beautiful form. I think he said, in a dream. He said, O Muhammad, do you know what the chiefs on high disputed about? I said, No. He put His hand between my shoulder-blades and I could feel its coolness on my chest (or just beneath my throat), then I knew everything that is in the heavens and on earth. He said, O Muhammad, do you know what the chiefs on high disputed about? I said, Yes. He said, About expiation, and expiation is staying in the mosque after prayers, walking to join congregational prayers, and doing wudoo properly in adverse conditions. Whoever does that will live a good life and die a good death, and will be as sinless as the day his mother bore him. He said, O Muhammad, when you pray, say Allaahumma innee asaluka fal al-khayraat wa tark al-munkaraat wa hubb al-masaakeen, wa idha aradta bi ibaadika fitnatan faqbudni ilayka ghayra maftoon (O Allaah, I ask you to make me do good deeds and avoid evil deeds, and to make me love the poor and wretched. If You want to test Your slaves, then take me unto You without subjecting me to the trial). Ones status in Paradise may be raised by spreading the greeting of salaam, feeding others, and praying at night when people are sleeping.(Reported by Imaam Ahmad, 16026; and by al-Tirmidhi, 3159, who said it is a saheeh hasan hadeeth).
Some of the scholars have commented that it is possible to see Allaah in a dream.
Imaam al-Daarimi said, in his refutation of Bishr ibn Ghiyaath, This seeing took place in a dream, and in a dream it is possible to see Allaah in any shape or form. (p. 166)
Shaykh al-Islam Ibn Taymiyah said, A believer may see his Lord in a dream, in various forms according to his faith and belief. If his eemaan is correct, he can only see Him in a beautiful form, and if his faith is lacking, this will be reflected in the way he sees Him. Seeing Allaah in a dream is not like seeing Him in reality. It may have different interpretations and meanings referring to something in reality.(al-Fataawa, 3/390)
He also said, Whoever sees Allaah in a dream sees Him in a form that corresponds to his own state. If he is righteous, he will see Him in a beautiful form, which is why the Prophet (peace and blessings of Allaah be upon him) saw him in the most beautiful form.(al-Fataawa, 5/251)
Shaykh Abd al-Azeez ibn Baaz was asked about the ruling concerning one who claims to have seen the Lord of Glory in a dream, and whether it was true, as some claim, that Imaam Ahmad ibn Hanbal had seen the Lord of Glory in his dreams more than one hundred times. The Shaykh answered as follows:
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) and others said that it is possible for a man to see his Lord in a dream, but what he sees is not reality, because there is nothing like Allaah, may He be glorified and exalted. Allaah says (interpretation of the meaning): There is nothing like unto Him, and He is the All-Hearer, the All-Seer. [al-Shoora 42:11]. Nothing in His creation is like unto Him. A person may dream that his Lord is speaking to him, and no matter what kind of image he sees, that image is not Allaah, because there is nothing that is like Allaah in any way. Shaykh Taqiy al-Deen (may Allaah have mercy on him) mentioned that dreams differ according to the state of the person who is dreaming. The more righteous and closer to goodness a person is, the more correct his dream will be, but the truth is still something other than what he sees, because the guiding principle is still that there is nothing like unto Allaah.
He may hear a voice and be told such-and-such, or to do such-and-such, without any clear image resembling anything in creation, because there is nothing at all like unto Allaah Some people may imagine that they have seen their Lord when in fact this is not the case. The Shaytaan may deceive a person and make him imagine that he is their Lord, as it was reported that he made Abd al-Qaadir al-Jeelaani see him on a throne above water, and said, I am your Lord and I freed you from doing any duties (worship, etc.). Abd al-Qaadir al-Jeelaani said, Get lost, O enemy of Allaah! You are not my Lord, because the commands of my Lord are not cancelled for anyone, or some similar words.
As for whether Imaam Ahmad saw his Lord, I do not know if this is true or not. It is said that he saw his Lord but I do not know if this is true. And Allaah knows best.