If the Imam forgets a prostration in the last rak'ah, must he repeat the prayer, or return, say "Allahu Akbar," and prostrate?

If, after concluding his prayer, the worshiper remembers that he missed a prostration in the last rak'ah, and not much time has passed, he must return to the prayer, even if he has turned away from the qiblah, to complete a full rak'ah as a make-up for the invalidated rak'ah. Then, he should perform the prostration of forgetfulness (sujood al-sahw) after the salutation. It is permissible to speak for the benefit of the prayer, but not for other reasons.