Musnad Ahmad 31:95

Isma'il narrated to us, al-Dastawa'i narrated to us, Yahya ibn Abi Kathir narrated to me, 'Iyad narrated to us, he said: I said to Abu Sa'id al-Khudri: 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 while he is sitting. And if Satan comes to one of you and says: 'You have broken your ablution,' let him say: 'You have lied,' unless he finds its odor with his nose or hears its sound with his ear."