Sahih Muslim Book 032, Hadith Number 6243.
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 Huraira reported that when this verse was revealed: "Whoever does evil will be requited for it", and when this was conveyed to the Muslims they were greatly perturbed. Thereupon Allah's Messenger (may peace be upon him) said: Be moderate and stand firm in trouble that falls to the lot of a Muslim (as that) is an expiation for him; even stumbling on the path or the pricking of a thorn (are an expiation for him). Muslim said that 'Umar b. Abd al-Rahman Muhaisin was from amongst the people of Mecca.