Praise be to Allah.
Many people have the habit of ending a recitation from the Quraan with the words Sadaqa Allaahu al-Azeem (Almighty Allaah has spoken the truth), but this has no basis in Islam because the Prophet (peace and blessings of Allaah be upon him) did not do it, nor was it the custom of the Sahaabah (may Allaah be pleased with them), and it was unknown among the Taabieen (the generation after the Sahaabah). This custom arose in later times because some reciters would say these words, on the basis of the aayah: Say: Allaah has spoken the truth [Aal Imraan 3:95 interpretation of the meaning], and people liked this. But this istihsaan (being liked) should be rejected, because if this was really something good, the Prophet (peace and blessings of Allaah be upon him), the Sahaabah and the Taabieen the salaf or first and best generations of the ummah would not have neglected to do it.
The aayah Say: Allaah has spoken the truth (Aal Imraan 3:95 interpretation of the meaning) does not mean that these words should be said at the end of any reading or recitation. If that were the case, He would have said, When you finish reading, say Allaah has spoken the truth, just as He said (interpretation of the meaning): So when you want to recite the Quraan, seek refuge with Allaah from Shaytaan (Satan), the outcast (the cursed one). [al-Nahl 16:98]
The aayah which the innovators use to support their practice of saying Sadaqa Allaah after reciting Quraan was actually revealed in the context of confirming what was said about how all food had been lawful to Bani Israeel except what Israeel had made unlawful for himself. Allaah said (interpretation of the meaning): Say (O Muhammad): Bring here the Tawraat (Torah) and recite it, if you are truthful. Then, after that, whosoever shall invent a lie against Allaah, such shall indeed be the zaalimoon (disbelievers). Say (O Muhammad), Allaah has spoken the truth; follow the religion of Ibraaheem (Islamic monotheism, i.e., he used to worship Allaah alone), and he was not of al-mushrikeen (polytheists). [Aal Imraan 3:93-95]
If this aayah meant that these words should be said after reciting from the Quraan, the first one to know and do this would have been the Messenger of Allaah (peace and blessings of Allaah be upon him). Because this is not the case, we know that this is not what was meant.
In conclusion, therefore, saying Sadaqa Allaahu al-Azeem after reciting Quraan is an innovation, and the Muslim should not say it.
But believing that Allaah has spoken the truth is obligatory, and whoever disbelieves or doubts the truth of what Allaah has said is a kaafir who is outside of the pale of Islam. We seek refuge with Allaah from that.
If a person says Allaah has spoken the truth on certain occasions, such as when something He has foretold comes to pass, affirming the truth of what He has said, then this is permissible, because something similar has been reported in the Sunnah. The Prophet (peace and blessings of Allaah be upon him) was giving a speech, and al-Hasan and al-Husayn came along, so he came down from the minbar, picked them up and put them in from of him, then he said: Indeed Allaah has spoken the truth: Your wealth and your children are only a trial [al-Taghaabun 64:15 interpretation of the meaning].