Praise be to Allaah. 

I do not know of any basis for the people’s habit of saying “Sadaqa Allaahu al-‘Azeem” when they finish reading Qur’aan, so it should not be taken as a habit. Indeed, according to the principles of sharee’ah it is more like bid’ah, if anyone believes that it is Sunnah. So this should not be done and should not be taken as a habit. 

With regard to the aayah (interpretation of the meaning):

“Say (O Muhammad): ‘Allaah has spoken the truth…’”[Aal ‘Imraan 3:95] –

This is not speaking about this matter. Rather Allaah was commanding him to explain to the people that Allaah had spoken the truth in what He had said in His Books, the Tawraat, etc., and that He had spoken the Truth in all that He had said to His slaves in the Tawraat, the Injeel and all other revealed Books. 

And He was speaking the truth in all that He said to His slaves in His Book the Qur’aan. But this is not evidence that it is mustahabb to say “Sadaqa Allaah al-‘Azeem” after reading the Qur’aan or after reading some aayahs or a soorah. This was not reported or known from the Prophet (peace and blessings of Allaah be upon him) or his companions (may Allaah be pleased with them). When Ibn Mas’ood recited to the Prophet (peace and blessings of Allaah be upon him) from the beginning of Soorat al-Nisaa’ until he reached the aayah (interpretation of the meaning):

“How (will it be) then, when We bring from each nation a witness and We bring you (O Muhammad) as a witness against these people?”[al-Nisaa’ 4:41] –

the Prophet (peace and blessings of Allaah be upon him) said to him, “Enough.” Ibn Mas’ood said: “I turned to him and saw that his eyes were filled with tears”, i.e., he was weeping because of the mention of this great status on the Day of Resurrection which is mentioned in this aayah, where Allaah says (interpretation of the meaning): “How (will it be) then, when We bring from each nation a witness and We bring you” – O Muhammad – “as a witness against these people?” – i.e., against his ummah. 

The point is that there is no basis in sharee’ah for adding these words – “sadaqa Allaah al-‘Azeem” – when finishing reading Qur’aan. What is prescribed is not to do this, in accordance with the example of the Prophet (peace and blessings of Allaah be upon him) and his companions (may Allaah be pleased with them). But if a person does that sometimes, without intending to, it doesn’t matter, for Allaah speaks the truth in all matters, may He be glorified and exalted. But making that a habit every time one reads Qur’aan, as many people do nowadays, has no basis, as stated above.