The Muslim has to control his tongue and not speak unless he says something good.
Al-Bukhari and Muslim narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever believes in Allah and the Last Day, let him speak good or else keep silent …”
Al-Nawawi (may Allah have mercy on him) said: “If he wants to speak, then if what he wants to say is good and he is confident that he will be rewarded for it, whether it is obligatory or praiseworthy, then let him speak, but if it does not seem to him that it is good and he will be rewarded for it, then let him refrain from speaking, whether he thinks it is haram (impermissible) or makrooh (disliked) or permissible. Based on this, it is recommended to refrain from saying permissible words and we are enjoined to avoid this, lest it lead us into something that is haram or makrooh, which is what happens in many cases or in most cases. Allah says (interpretation of the meaning):
‘Not a word does he (or she) utter but there is a watcher by him ready (to record it).’ [50:18]
Al-Imam al-Shafi’i understood the hadeeth (narration) to mean that if a person wants to say something, he should think about it: if it seems to him that it will not do any harm, then he should speak, but if he thinks that it will do some harm or he thinks that this is most likely, then he should refrain from speaking.”
The Prophet (peace and blessings of Allah be upon him) commanded us to control our tongues in more than one hadeeth, such as that narrated by al-Tirmidhi from ‘Uqbah ibn ‘Amir (may Allah be pleased with him) who said: “I said, ‘O Messenger of Allah, what is salvation?’ He said, ‘Control your tongue, keep to your house and weep over your sin.’” Classed as saheeh (authentic) by al-Albani.
Al-Tirmidhi also narrated that the Prophet (peace and blessings of Allah be upon him) said to Mu’adh (may Allah be pleased with him), after teaching him some of the laws of Islam, “Shall I not tell you what is the foundation of all that?” I [Mu’adh] said: “Of course, O Prophet of Allah.” He took hold of his tongue and said, “Control this.” I said, “O Prophet of Allah, will we be held responsible for what we say with it?” He said, “May your mother be bereft of you, O Mu’adh! Will the people be thrown into Hell on their faces or on their noses for anything other than the harvest of their tongues?” Classed as saheeh by al-Albani.
The hadeeth enjoins refraining from indulging in specific things, because this is something that is of no benefit to a person, and indeed it may cause him a great deal of harm in his spiritual and worldly affairs.
It was narrated from Ibn Mas’ood (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “If my Companions are mentioned, then keep quiet; if the stars are mentioned, then keep quiet; and if the divine decree is mentioned, then keep quiet.” (Narrated by al-Tabarani and classed as saheeh / authentic).
This hadeeth indicates that it is not permissible for a person to speak badly of the Companions of the Prophet (peace and blessings of Allah be upon him), and that he should keep quiet about the disputes that occurred among them, because speaking badly of them or criticizing them implies that one is rejecting what Allah said about them in the Quran, where He praised them by saying (interpretation of the meaning):
“And the foremost to embrace Islam of the Muhajiroon (emigrants) and the Ansar (helpers) and also those who followed them exactly (in faith). Allah is well-pleased with them as they are well-pleased with Him. He has prepared for them gardens under which rivers flow (Paradise), to dwell therein forever. That is the supreme success.” [9:100]
And Allah says (interpretation of the meaning):
“Muhammad is the Messenger of Allah. And those who are with him are severe against disbelievers, and merciful among themselves. You see them bowing and falling down prostrate (in prayer), seeking bounty from Allah and (His) good pleasure. The mark of them (i.e. of their faith) is on their faces (foreheads) from the traces of prostration (during prayers).”
This is how Allah has described them in His Book, so no doubt they are of just character and are the best and greatest of mankind. So no one hates them but a hypocrite and no one loves them but a believer.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
“One of the basic principles of Ahl al-Sunnah wa’l-Jama’ah (followers of prophetic tradition) is that they think and say nothing to criticize the Companions of the Messenger of Allah (peace and blessings of Allah be upon him), as Allah has described them in the verse in which He says (interpretation of the meaning):
‘And those who came after them say: Our Lord! Forgive us and our brethren who have preceded us in faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful.’
And they obey the command of the Messenger of Allah (peace and blessings of Allah be upon him): ‘Do not curse my Companions, for by the One in Whose hand is my soul, if any one of you were to spend the equivalent of Mount Uhud in gold, he would not reach the status of one of them, nor even come half way.’ (Agreed upon).” Majmoo’ al-Fataawa, 3/152.
Abu Zar’ah (may Allah have mercy on him) said:
“If you see a man criticizing any one of the sahabah (Companions), then know that he is a heretic, because the Quran is true, and the Messenger is true, and what he brought is true, and no one conveyed all of that to us but the sahabah. So whoever criticizes them means, in effect, to say that the Quran and Sunnah (prophetic teachings) are false, so it is more appropriate that he should be criticized and ruled to be a heretic who is misguided.”
Al-Sawa’iq al-Muhriqah ‘ala Ahl al-Rafd wa’l-Dalal wa’l-Zandaqah.
With regard to the second part of the hadeeth, which enjoins refraining from speaking about the stars, what is referred to by that – and Allah knows best – is seeking guidance by the stars concerning matters that are unseen, as the people of the jahiliyyah (pre-Islamic days of ignorance) used to do by means of astrology, such as using the movement of heavenly bodies to indicate when certain earthly events would occur, such as the blowing of the wind, rainfall, changes in prices, and other matters which they claimed to know from the movements of heavenly bodies. They used to say that whoever got married under a particular star, then such and such would happen to him, or whoever traveled under a particular star, such and such would happen to him, and whoever was born under a particular star would be lucky or unlucky, etc.
(See Kitab al-Tawheed by al-Shaykh Salih al-Fawzan. See also Fatawa al-‘Aqeedah by al-Shaykh Ibn ‘Uthaymeen, where he makes some very useful comments).
With regard to the third part of the hadeeth, which refers to refraining from speaking about the divine decree (al-qadar), Abu Ja’far al-Tahhawi (may Allah have mercy on him) said:
“The divine decree is a secret known which Allah has hidden from His creation. No angel who is close to Him and no Prophet whom He sent knows that. Pursuing such matters only leads to failure and deprivation of the mercy of Allah, and it leads to injustice, so beware of looking into the matter, thinking about it and wondering about it. Allah has kept knowledge of His decree from His slaves, and has forbidden them to pursue the matter, as He says in His Book (interpretation of the meaning):
‘He cannot be questioned as to what He does, while they will be questioned.’
So whoever questions what Allah does has rejected the ruling of the Book, and whoever rejects the ruling of the Book is a kafir (disbeliever).” Sharh al-‘Aqeedah al-Tahhawiyyah.
So the Muslim has to submit to Allah in all His affairs, and know that whatever happens to him could never have missed him, and what misses him could never have happened to him. There is a lot that may be said on this topic. May Allah bless the man who believes in the will and decree of Allah without indulging in philosophical discussion about the matter.