Is it Bid'ah to say "Sadaqa Allahu al-Adheem" after reading Qur'an?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no doubt that Allah is The Most Wholly Truthful One.
He Says (interpretation of meaning): {and whose words can be truer than those of Allâh? (Of course, none).} [4:122].
As for one's saying "Allah The Great has told the truth", it is a form of an unrestricted Zikr. So, pronouncing such a form at a specific time, place or state is baseless, i.e. without an evidence from the Qur'an or Sunnah. Thus, we state that it is not permissible to use such a form of Zikr after reciting the Qur'an. In other words, it is a form of Bid'ah, since it is not supported by any evidence from the Sunnah.
So, a reciter is advised to keep silent at the end of recitation for the sound narration that reports the Prophet's (Sallallahu Alaihi wa Sallam) saying: "That is enough" when the Prophet (Sallallahu Alaihi wa Sallam) wanted Ibn Masoud to stop recitation. [ al-Bukhari ].
This means that a Muslim is required to search for the Sunnah of the Prophet (Sallallahu Alaihi wa Sallam) as well as to follow it, not to innovate, in word or in deed. But, we think that such forms result from one's being far from circles of knowledge. This makes students repeat such a form "Allah has told the truth" thinking that is the Sunnah, while it is not.
Allah knows best.