Musannaf Ibn Abi Shaybah 4463 — A man who forgets a prostration

Mujahid was asked: A man becomes doubtful whether he made a prostration or not, and he is now sitting; what should he do? He said: If you wish, make the prostration, and then, when you have completed the prayer, make the two prostrations of forgetfulness while sitting. And if you wish, do not make it, and make two prostrations while sitting at the end of the prayer.