narrated aisha (radi-allahu 'anha):
allah's apostle (sallallahu 'alaihi wa sallam) heard a man reciting the qur'an at night, and said, "may allah bestow his mercy on him, as he has reminded me of such-and-such verses of such-and-such suras, which i was caused to forget."
bukhari vol. 6 : no. 558