Musnad Abu Ya'la al-Mawsili 42:268

Abu Khaythama narrated to us, Isma'il bin Ibrahim narrated to us, from al-Dastawa'i, from Yahya bin Abi Kathir, from 'Iyad, who 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 (ﷺ) 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 while he is in his prayer and says, 'You have broken your ablution,' let him say, 'You have lied,' unless he finds its odor with his nose or hears a sound with his ear."