And to have eemaan that when a man becomes ill, Allaah gives him reward for his illness.
Allaah does not allow the reward due to the believers to become lost and He causes afflictions to strike the believers in order to purify them or to multiply their reward. He may cause calamities to happen to the believer to wipe away his sins and to purify him from sins. And it may be the case that he does not have sins but He still causes them to happen to him in order to raise his ranks in the Hereafter because Allaah has written for him a high rank in Paradise, which he cannot reach through his deeds. So, therefore Allaah tries him with afflictions so that He multiplies the reward for him so that he can reach this level. So the believer is upon good and therefore he sallAllaahu `alayhi wa sallam said:
“How amazing is the matter of the believer, all of his matters are good for him. If something pleasing comes upon him then he gives thanks and that will be good for him and if something harmful comes to him then he has patience (sabr) and that will be good for him and that will not be except for the believer (mu’min).”
So indeed the believer is struck by afflictions and they are something beneficial for him for either Allaah will wipe away his sins with them or otherwise Allaah will raise his rank on account of them.
 Reported by Muslim (2,999) from a hadeeth of Suhayb radiyAllaahu `anhu.
 Translator’s Side Point: Some of the People of Knowledge mentioned that if a person is struck by affliction and he has patience upon it then he is rewarded. And if he doesn’t have patience, shows displeasure and the like then that’s something different. W`Allaahu A`lam.