Musannaf Ibn Abi Shayba 3:2090

Abu Khalid al-Ahmar narrated to us, from Hisham, from Ibn Sirin, from Abu Hurairah, who said: The Prophet (peace and blessings of Allah be upon him) performed the two prostrations of forgetfulness after he had said the taslim. He said the takbir and prostrated, then said the takbir while he was sitting. Then he raised his head and said the takbir, then he prostrated and said the takbir, then he raised his head and said the takbir.