Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) led one of the daytime prayers. When he (peace and blessings be upon him) turned away from the prayer, the Companions (may Allah be pleased with them) said: O Messenger of Allah! You prayed five rak'ahs. He (peace and blessings be upon him) said: "I am a human being like you; I remember just as you remember, and I forget just as you forget."
Then he turned back and performed the two prostrations of forgetfulness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3841