What did Allah's Prophet (saw) advise when a person is unsure whether they have prayed the correct amount of rak`at?
 Narrated Ibn Mas`ud:                     that Allah's Prophet led them in the Zuhr prayer and he offered either more or less rak`at, and it was said to him, "O Allah's Apostle ! Has the prayer been reduced, or have you forgotten?" He asked, "What is that?" They said, "You have prayed so many rak`at." So he performed with them two more prostrations and said, "These two prostrations are to be performed by the person who does not know whether he has prayed more or less (rak`at) in which case he should seek to follow what is right. And then complete the rest (of the prayer) and perform two extra prostrations."