Musannaf Ibn Abi Shayba 3:2023

Mu'tamir narrated to us, from Layth, from Mujahid, regarding a man who doubts a prostration while he is sitting, not knowing whether he performed it or not. Mujahid said: If you wish, perform it, and when you have finished your prayer, perform two prostrations while you are sitting. And if you wish, do not perform it, and perform two prostrations while you are sitting at the end of your prayer.