Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.001A.

Anyone suffering from an illness should remain patient, for there is no reward better or more enriching than that reserved for those who endure in patience.

Suhaib ibn Sinan narrated that the Prophet, peace be upon him, said: “How remarkable is the case of the believer! There is good for him in everything, but this is not the case for anyone except for the believer. When the believer receives any good, he is thankful to Allah, and gets a reward. And when some misfortune befalls him, he endures it patiently, for which he is (also) rewarded.”

Anas narrates: “I heard the Prophet, peace be upon him, saying: ‘Allah says: “When I afflict a servant of mine with respect to his two most beloved things (meaning his eyes), and he endures it patiently, I grant him paradise in return’.”

‘Ata ibn Rabah related that he heard Ibn ‘Abbas say: “Shall I show you a woman of Paradise?” I said: “Yes, indeed.” He said: “A black woman came to the Prophet, peace be upon him, and said: ‘I suffer from epileptic fits, and because of these, (at times) my body becomes uncovered. Would you invoke Allah, the Exalted One, to cure me of this disease?’ The Prophet, peace be upon him, said: ‘If you wish, you can be patient and you will attain Paradise (for this suffering). But if you prefer, I will pray to Allah, the Exalted, to cure you of it?’ The woman said: ‘I will be patient,’ then added: ‘I become uncovered (when I have fits), so invoke Allah for me that I do not become uncovered.’ So the Prophet, peace be upon him, prayed for her.”