Jami` at-Tirmidhi 398
جامع الترمذي#398SahihAbdur-Rahman bi Awf narrated that he heard the Prophet (S) saying:"When one of you becomes forgetful in his Salat so that he does not know if he prayed one or two, then let him consider it one. When he does not know if he prayed three or four, then let him consider it three, and let him perform two prostrations before he says the Taslim
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiHasan
In-book reference: 2:251