Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that he heard the Prophet (SAW) say, "When one of you forgets in his salah and does not recall whether he has prayed one or two then let him settle for one, but if he does not recall whether he has prayed two or three then let him settle for two, but if he does not know whether he has prayed three or four then let him settle for three. And he should prostrate two prostrations before he makes the salutation." [Ahmed 1656, Ibn e Majah 1209]