Sahih Muslim Book 32, Hadith Number 6241.

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

‘Aisha reported: I heard Allah’s Messenger (may peace be upon him) as saying: There is nothing (in the form of trouble) that comes to a believer even if it is the pricking of a thorn that there is decreed for him by Allah good or his sins are obliterated.