What is the Ruling on saying “Sadaqa Allahu Al Adheem” after reading the Qur'an?

Whoever says it in order to get closer to Allah through this expression and he assumes it is Sunnah, then he has made a mistake, because saying it was not known after the reading of Qur'an at the time of the Prophet ﷺ. ‏And it was not known to be said after reading during the time of his Companions, nor the time of the Tabieen. ‏So seeking closeness to Allah specifically with this phrase is a type of innovation. ‏And some innovations are severe, and some are lesser than that. However, the goodness is in staying away from it all.