Musannaf Ibn Abi Shaybah 4473 — A man who prays and does not know

Salim said: If someone is in doubt whether he has prayed three rak'ahs or four, let him rid himself of the doubt and make the prostrations of forgetfulness. Yahya bin Sa'id said: I mentioned this statement to al-Qasim, and he said, "I say the same too; I say the same too."