Ibrahim bin Suwayd (may Allah have mercy on him) said: Alqamah (may Allah have mercy on him) led us in five rak'ahs. It was said to him (that he had prayed five rak'ahs), and he said: I did not do that. I said: Indeed, that is what happened. He said: You too, O A'war? I said: Yes. So he performed two prostrations. Then he told us, from Abdullah (bin Mas'ud) (may Allah be pleased with him), from the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) prayed five rak'ahs. The Companions began whispering among themselves and said: Has the prayer been increased? He (peace and blessings be upon him) said: "No." They informed him (peace and blessings be upon him) (of what had happened), so he (peace and blessings be upon him) folded his leg and performed two prostrations, then said: "I am only a human being; I forget just as you forget."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9810