(a) Ibrahim bin Suwayd (may Allah have mercy on him) narrates that Alqamah (may Allah have mercy on him) led us in the Zuhr prayer with five rak'ahs. When he gave the salam, the people said: O Abu Shibl! You have prayed five rak'ahs. He said: No, I did not do so. The people said: Yes indeed, you did. Ibrahim bin Suwayd (may Allah have mercy on him) said: I was at one side of the people, and I was a boy at the time. I said: Sir! You did indeed pray five rak'ahs. He said: You too say so, O one-eyed one? I said: Yes. So he turned back towards the qiblah and performed two prostrations, then gave the salam. Then he said: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) led us in a prayer of five rak'ahs. When he (peace and blessings be upon him) gave the salam, the people whispered among themselves, and someone said: O Messenger of Allah! Has something been added to the prayer? He (peace and blessings be upon him) said: "No." The Companions (may Allah be pleased with them) said: But you have led us in five rak'ahs. So he (peace and blessings be upon him) turned back and performed two prostrations, then, after giving the salam, he said: "I am only a human being like you; I forget just as you forget. So when any of you forgets in the prayer, let him perform two prostrations."
(b) These are the words of Jarir's narration. Imam Muslim (may Allah have mercy on him) recorded it in his Sahih, but the wording there differs slightly; there it reads: "When any of you forgets, let him perform two prostrations." This was narrated by Ibn Numayr (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3840