Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
There are no reports from the Prophet (Sallallahu Alaihi Wasallam), Sahabah and the Tabi'een (successors) regarding this practice. Therefore, Du'a after Salatul Janazah is Bid'ah and not permissible. (Ahsanul Fatawa Vol. 1 Pg. 336)