It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) led the Zuhr or Asr prayer. When he (peace and blessings be upon him) completed the prayer, it was said: O Messenger of Allah, has something new occurred in the prayer? He said: "What is that?" They said: You (peace and blessings be upon him) led five rak'ahs. So he (peace and blessings be upon him) faced the qiblah and performed two prostrations, then said: "I am only a human being; I forget as you forget. So when one of you has an uncertainty in his prayer, let him perform two prostrations."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9802