narrated abu sa'id al-khudri and abu huraira (radi-allahu 'anhum):
the prophet (sallallahu 'alaihi wa sallam) said, "no fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a muslim, even if it were the prick he receives from a thorn, but that allah expiates some of his sins for that."
bukhari vol. 7 : no. 545