Narrated Abu Huraira (Radi-Allahu 'anhu):
The Prophet (Sallallahu 'Alaihi Wa Sallam) said, "When the call for the prayer is pronounced, Satan takes to his heels, passing wind with noise, When the call for the prayer is finished, he comes back. And when the Iqama is pronounced, he again takes to his heels, and after its completion, he returns again to interfere between the (praying) person and his heart, saying to him. 'Remember this or that thing.' till the person forgets whether he has offered three or four Rakat: so if one forgets whether he has prayed three or four Rak'a-t, he should perform two prostrations of Sahu (i.e. forgetfulness)."
Bukhari Vol. 4 : No. 505