Is saying "صدق الله العظيم" (Allah the Almighty has spoken the truth) at the end of every Surah an innovation?

The statement "God Almighty has spoken the truth" is an absolute ذكر (remembrance/invocation). Restricting it to a specific time, place, or situation requires evidence, because it was not narrated from the Prophet, peace and blessings be upon him. Worshipping in a way that has not been legislated is not permissible and is considered an innovation. The best practice upon finishing a recitation is simply to remain silent, as the Prophet, peace and blessings be upon him, said to Ibn Mas'ud: "Enough for you."