What is the ruling on saying "Ṣadaqa Allāhu al-ʿAẓīm" (Allah, the Most High, has spoken the truth) after completing the recitation of the Holy Quran, and is this considered permissible, taking into account that it was not said during the time of the Prophet, peace be upon him?

Allah Almighty is the most truthful of speakers, and believing Him is an obligation. Whoever disbelieves Him or doubts the truth of what He has conveyed is a disbeliever. It is permissible to say "Allah has spoken the truth" on occasions that affirm the truthfulness of Allah's word, as the Prophet, peace and blessings be upon him, did. However, the consistent utterance of "Allah, the Most Great, has spoken the truth" after reciting the Quran is considered an innovation (bid'ah) by contemporary verifying scholars, because it has not been authentically reported that the Prophet, peace and blessings be upon him, nor his companions, nor the Tabi'un (followers) consistently did so. Prescribed remembrances (adhkar) can only be established by proof, and worshipping in a manner that has not been legislated is impermissible. Nevertheless, if a person says it occasionally for reasons that necessitate it, there is no harm.