Musannaf Ibn Abi Shayba 3:2057

Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, who said: I asked Muhammad about something regarding forgetfulness in a voluntary prayer. He said: "I do not know where its place is." I said: "Should I perform two prostrations after it?" He said: "Are you likening it to the obligatory prayer? As for me, had I done so, I would not have."