Musnad Ahmad 31:781

Musa ibn Dawud narrated to us, Sulayman ibn Bilal narrated to us, from Zayd ibn Aslam, from ‘Ata’ ibn Yasar, from Abu Sa‘id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you is in doubt regarding his prayer, and does not know how much he has prayed, three or four, then let him cast aside the doubt and build upon what he is certain of. Then let him prostrate twice before he gives the salutation. If he has prayed five, they will be an even-numbering for his prayer." Musa said once: "If he has prayed five, they will make his prayer even, and if he has prayed the completion of four, they will be a humiliation for the Shaytan."