Maliks Muwatta Book 3, Hadith Number 68.
Section : Completing what is Recalled When Uncertain how much has been Prayed.
Yahya related to me from Malik from Afif ibn Amr as-Sahmi that Ata ibn Yasar said, “I asked Abdullah ibn Amr ibn al-As and Kab al Ahbar about someone who was uncertain in his prayer, and did not know whether he had prayed three or four rakas. Both of them said, ‘He should pray another raka and then do two sajdas from the sitting position.'”