Musnad Ahmad 30:2649

Waki' narrated to us, he said: Malik ibn Anas narrated to me, from Dawud ibn al-Husayn, from Abu Sufyan, the freed slave of Ibn Abi Ahmad, from Abu Hurayrah: "That the Messenger of Allah (peace and blessings of Allah be upon him) prayed with them, and he became forgetful. When he finished the prayer (with the taslim), he prostrated two prostrations, then he finished the prayer (with the taslim)."