Nahj al-Balagha (Peak of Eloquence)

42. Amīr al-mu’minīn, peace be upon him, said to one of his companions during his sickness: May Allāh make your illness a means for writing off your sins, because there is no reward for sickness but that it erases sins and makes them fall like (dried) leaves. Reward lies in saying by the tongue and doing something with the hands and feet. Certainly, Allāh, the Glorified, admits into Paradise by virtue of truthfulness of intention and chastity of heart to whomsoever He wishes from among His creatures. As-Sayyid ar-Raḍī says: Amīr al-mu’minīn is right in saying that there is no reward for sickness as such because compensation is admissible in respect of the acts of Allāh, the Sublime, towards his creatures such as grief, illness and the like, whereas reward and recompense becomes admissible against actions by the creature. This is the difference between the two and Amīr al-mu’minīn has clarified it through his lustrous knowledge and sound view.