What did the Messenger of Allah (peace be upon him) do when he forgot something while performing salah?
 'Abdullah (b. Mas'ud) reported:                      The Messenger of Allah (may peace be upon him) said prayer and he omitted or committed (something). Ibrahim (one of the narrators of this hadith) said: It is my doubt, and it was said: Messenger of Allah, has there been any addition to the prayer? He (the Holy Prophet) said: Verily I am a human being like you. I forget just as you forget so when any one of you forgets, he must perform two prostrations, and he (the Holy Prophet) was sitting and then the Messenger of Allah (may peace be upon him) turned (his face towards the Qibla) and performed two prostrations.