Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1022
Narated By Ata' ibn Yasar : The Prophet (pbuh) said: If one of you is in doubt about his prayer, and if he is sure that he has prayed three rak'ahs, he should stand and complete one rak'ah along with its prostrations. Then he should sit and recite the tashahhud. When he finishes the prayer, and there remains nothing except salutation, he should make two prostrations while he is sitting and afterwards should give the salutation.