Musnad Abu Ya'la al-Mawsili 42:168

Sufyan ibn Waki' narrated to us, my father narrated to us, from Ali ibn al-Mubarak, from Ibn Abi Kathir, from Iyad, who said: I asked Abu Sa'id al-Khudri and said: "One of us prays and does not know how much he has prayed." He said: The Messenger of Allah (ﷺ) said to us: "If one of you prays and does not know how much he has prayed, let him perform two prostrations of forgetfulness 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 a scent or hears a sound with his ear."