Sahih Bukhari Volume 002, Book 022, Hadith Number 313.
Narated By Abu Huraira : Allah's Apostle said, "When the Adhan for the prayer is pronounced, then Satan takes to his heels passing wind so that he may not hear the Adhan and when the Muadh-dhin finishes, he comes back; and when the Iqama is pronounced he again takes to his heels and when it is finished, he again comes back and continues reminding the praying person of things that he used not to remember when not in prayer till he forgets how much he has prayed." Abu Salama bin 'Abdur-Rahman said, "If anyone of you has such a thing (forgetting the number of Rakat he has prayed) he should perform two prostrations of Sahu (i.e. forgetfulness) while sitting." Abu Salama narrates this from Abu Huraira.