Musnad Ahmad 31:793

Yazid and Abu al-Nadr narrated to us, they said: Abd al-Aziz ibn Abd Allah ibn Abi Salamah informed us, from Zayd ibn Aslam, from Ata ibn Yasar, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you is in doubt during prayer and does not know whether he has prayed three or four, let him stand and pray a rak'ah." Yazid said: "Until the doubt is regarding an addition." "Then let him perform two prostrations of forgetfulness. If he had prayed five, they will make his prayer even for him, and if he had prayed four, they will humiliate the devil."