Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"I heard the Messenger of Allah (saw) say: 'If anyone of you is uncertain as to whether he has prayed one or two Rakah, let him assume it is one. If he is uncertain as to whether he has prayed two or three, let him assume it is two. If he is uncertain as to whether he has prayed three or four, let him assume it is three. Then let him complete what is left of his prayer, so that the doubt will be about what is more. Then let him prostrate twice while he is sitting, before the Taslim (saying the Salam).'" Sahih