Sahih Muslim Book 32, Hadith Number 6242.

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.

Abu Sa’id and abu Huraira reported that they heard Allah’s Messenger (may peace be upon him) as saying: Never a believer is stricken with discomfort, hardship or illness, grief or even with mental worry that his sins are not expiated for him.