Musnad Ahmad 26:23

Sufyan narrated to us, from Mansur, from Ibrahim, from Alqamah, from Abdullah: That the Prophet, peace and blessings of Allah be upon him, performed them (the two prostrations) after the Taslim. And he said once: "The Prophet, peace and blessings of Allah be upon him, performed the two prostrations for forgetfulness after the Taslim."