Muslim scholars say that saying “Sadaqa Allahu Al-`Azim” after reciting the Qur’an or when listening to it is not a blameworthy bid`ah (innovation in religion).
In this regard, Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, states: “Saying “Sadaqa Allahu Al-`Azim” after reciting the Qur’an or when listening to it is not a blameworthy bid`ah due to the following reasons:
1. No evidence has been reported to forbid it.
2. It is a kind of dhikr (remembrance of Allah), which is recommended at any time.
3. Some scholars have spoken of the virtue of uttering this expression and considered it one of the ethics of recitation. They even maintain that saying it in the salah does not break it.
4. These words or the like have been mentioned in the Qur’an: “Say: Allah has spoken the truth (Sadaqa Allahu), therefore follow the religion of Ibrahim, the upright one; and he was not one of the polytheists.” (Al `Imran: 95) “And when the believers saw the allies, they said: this is what Allah and His Messenger promised us, and Allah and His Messenger spoke the truth (wa sadaqa Allahu wa rasulluh); and it only increased them in faith and submission.” (Al-Ahzab: 22)

Al-Qurtubi prefaced his Tafsir by quoting Al-Hakim At-Tirmidhi as saying that among the ethics of reciting the Qur’an is for one to say, upon finishing the recitation, “True are the words of Almighty Allah” or a similar expression of the same meanings, testifying to the truth of all what Allah says (in the Qur’an), and bearing witness that the Messenger (peace and blessings be upon him) has conveyed this message to mankind. An example of this is to say, “Almighty Allah has spoken the truth and His honorable Messenger has conveyed it.” And then to confirm it by saying, “You have spoken the truth, O our Lord; You sent it (i.e., the message) to Your Messengers, and we testify that this is true. O Allah, make us of those who testify to the truth, and then the reciter is supposed to make du`a’ with whatever he (or she) wishes.

In Fiqh Al-Madhahib Al-Arba`ah, the Egyptian Awqaf edition, it is stated: “If one says in prayer words like ‘Sadaqa Allahu Al-`Azim’ when the reciter finishes his reading, this does not break the salah if one intends to just glorify Allah, the Almighty, or make mention of Him. The Shafi`is maintain that salah is not invalidated at all by saying these words.”

Having said this, I wonder how some people dare consider ‘Sadaqa Allahu Al-`Azim’ after reciting the Qur’an as bid `ah. We should be very careful when giving any Islamic rulings, as Allah says in the Qur’an: “And, for what your tongues describe, do not utter the lie, (saying) this is lawful and this is unlawful, in order to forge a lie against Allah; surely those who forge the lie against Allah shall not prosper.” (An-Nahl: 116)”