Musannaf Ibn Abi Shayba 3:2002

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mughira, who said: I said to Ibrahim, "I recited [the chapter of] al-Sajdah and prostrated during it, then I added another prostration to it out of forgetfulness." He said: "Perform the two prostrations of forgetfulness."