Yahya related to me from Malik from Afif ibn Amr as-Sahmi thatAta ibn Yasar said, "I asked Abdullah ibn Amr ibn al-As and Kab alAhbar about someone who was uncertain in his prayer, and did not knowwhether he had prayed three or four rakas. Both of them said, 'Heshould pray another raka and then do two sajdas from the sittingposition.' " (IT)
App reference (IT): Book 3, Hadith 68 (IT)
USC-MSA web (English) reference: Book 3, Hadith 68 (IT)
Arabic reference (IT): Book 3, Hadith 215 (IT)