Musnad Ahmad 26:732

Ibn Idris narrated to us, he said: I heard al-Hasan ibn Ubayd Allah mentioning from Ibrahim, from Alqama, that he informed them from Abdullah: That the Messenger of Allah (peace and blessings of Allah be upon him) prayed five [rak'ahs] with them, then he turned away. Some of the people began whispering to one another, and they said to him: "O Messenger of Allah, you prayed five." So he turned away and performed two prostrations with them, then he gave the salutation, and said: "I am only a human being; I forget just as you forget."