Musannaf Ibn Abi Shayba 3:2152

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from al-Hasan ibn Ubayd Allah, from Ibrahim: That he would perform the two prostrations of forgetfulness (sajdatay al-sahw), and we would not know of him having omitted anything, so we would say, "You did not omit anything." He would say, "I spoke to myself about something."