Maliks Muwatta Book 3, Hadith Number 67.
Section : Completing what is Recalled When Uncertain how much has been Prayed.
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.”