Yahya related to me from Malik from Umar ibn Muhammad ibn Zaydfrom Salim ibn Abdullah that Abdullah ibn Umar used to say, "If youare uncertain in the prayer, estimate what you think you haveforgotten of the prayer and repeat it, then do the two sajdas offorgetfulness from the sitting position."
App reference: Book 3, Hadith 67
USC-MSA web (English) reference: Book 3, Hadith 67
Arabic reference: Book 3, Hadith 214