Yahya related to me from Malik from Zayd ibn Aslam from Ata ibn Yasar that the Messenger of Allah, may Allah bless him and grant him peace, said, "If you are uncertain in the prayer and do not know whether you have prayed three or four rakas, then pray a raka and make two prostrations from the sitting position before the taslim. If the raka that you prayed was the fifth, then you make it even by these two sajdas, and if it was the fourth, then the two prostrations spite Shaytan."
Muwatta Malik Book 3, Hadith 66
← Previous Hadith Yahya related to me from Malik from Ibn Shihab from Said ibn al- Musayyab, and from Abu Salama ibn Abd ar-Rahman, the same as that.

Malik said, "Every forgetfulness which decreases from the prayer, prostrations for it come before the greeting, and every forgetfulness which is an addition to the prayer, prostrations for it come after the greeting."

Muwatta Malik Book 3, Hadith 65 Next Hadith → Yahya related to me from Malik from Umar ibn Muhammad ibn Zayd from Salim ibn Abdullah that Abdullah ibn Umar used to say, "If you are uncertain in the prayer, estimate what you think you have forgotten of the prayer and repeat it, then do the two sajdas of forgetfulness from the sitting position."

Muwatta Malik Book 3, Hadith 67