Sahih Muslim Book 32, Hadith Number 6240.

Chapter : Whenever a believer falls sick or is stricken with grief or something like it, there is a reward for him even if it is pricking of a thorn.

‘Aisha said: No trouble comes to a believer even if it is the pricking of a thorn that it becomes (the means) whereby his sins are effaced or his sins are obliterated. Yazid says: He does not know which word ‘Urwa said (whether he said Qussa or Kuffira).