Musnad Ahmad 31:509

Yahya narrated to us, from Hisham, Yahya narrated to us, from Iyad, that he asked Abu Sa’id al-Khudri, saying: One of us prays and does not know how much he has prayed? He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you prays and does not know how much he has prayed, let him prostrate two prostrations. If the devil comes to him and says: 'You have broken your ablution,' let him say: 'You have lied,' unless he finds a smell with his nose or a sound with his ear."