Musannaf Ibn Abi Shaybah 4475 — A man who prays and does not know

Ata bin Yasar said: I asked Sayyiduna Abdullah bin Amr (may Allah be pleased with him) and Ka'b: If a man is in doubt whether he has prayed three rak'ahs or four, what should he do? Both of them said: He should pray one rak'ah and then, at the end, make the prostrations of forgetfulness while sitting.