It was narrated to me by Muhammad ibn Ahmad ibn Abi Khalaf, who said: It was narrated to us by Musa ibn Dawud, who said: It was narrated to us by Sulayman ibn Bilal, from Zayd ibn Aslam, from 'Ata' ibn Yasar, from Abu Sa'id al-Khudri, who said: The Messenger of Allah, peace and blessings be upon him, said: "If one of you has doubt during his prayer and does not know how many he has prayed, whether three or four, let him cast aside the doubt and build upon what he is certain of. Then let him prostrate two prostrations before he gives the salutation. If he has prayed five, they will make his prayer even; and if he has prayed a complete four, they will be a humiliation for Satan."