Musannaf Ibn Abi Shayba 3:2034

Zayd ibn al-Hubab narrated to us, he said: Malik ibn Anas narrated to me, from Afif ibn Amr al-Sahmi, from Ata ibn Yasar, he said: I asked Abdullah ibn Amr ibn al-As and Ka’b about the one who is in doubt regarding his prayer, whether he prayed three or four. Both of them said: Let him stand and pray a rak’ah, then let him prostrate two prostrations while he is sitting.