"[Illness] provides you the ability to grow, to evolve, to mature."

67:1-2 Blessed be He in Whose hands is Dominion; and He over all things hath Power;- He Who created Death and Life, that He may try which of you is best in deed: and He is the Exalted in Might, Oft-Forgiving;-

2:155-157 Be sure We shall test you with something of fear and hunger, some loss in goods or lives or the fruits (of your toil), but give glad tidings to those who patiently persevere, who say, when afflicted with calamity: "To Allah We belong, and to Him is our return": They are those on whom (descend) blessings from Allah, and Mercy, and they are the ones that receive guidance.

-The Apostle of Allah (peace be upon him) visited me while I was sick. He said: Be glad, Umm al-Ala' for Allah removes the sins of a Muslim for his illness as fire removes the dross off gold and silver. [Sunan Abu Dawood Book 20, Number 3086]

-"The more severe the trial and hardship, the greater its reward. When God cherishes love for a group of people, He puts them to trial to cleanse them more and to make them pure. Hence those who acquiesce in the Will of God win the pleasure of God. And those who harbour resentment against God in the hour of trial, God also is displeased with them. [Tirmidhi]

-" No fatigue, nor disease, nor sorrow, nor sadness, nor hurt, nor distress befalls a Muslim, even if it were the prick he receives from a thorn, but that Allah expiates some of his sins for that." [Sahih Bukhari Volume 7, Book 70, Number 545]

-"There is nothing (in the form of trouble) that comes to a believer even if it is the pricking of a thorn that there is decreed good for him by Allah or his sins are obliterated. [Sahih Muslim Book 32, Number 6241]

-"The example of a believer is that of a fresh tender plant; from whatever direction the wind comes, it bends it, but when the wind becomes quiet, it becomes straight again. Similarly, a believer is afflicted with calamities (but he remains patient till Allah removes his difficulties.) And an impious wicked person is like a pine tree which keeps hard and straight till Allah cuts (breaks) it down when He wishes." [Sahih Bukhari Volume 7, Book 70, Number 547]

-"If Allah wants to do good to somebody, He afflicts him with trials." [Sahih Bukhari Volume 7, Book 70, Number 548]

-I visited the Prophet during his ailments and he was suffering from a high fever. I said, "You have a high fever. Is it because you will have a double reward for it?" He said, "Yes, for no Muslim is afflicted with any harm but that Allah will remove his sins as the leaves of a tree fall down." [Sahih Bukhari Volume 7, Book 70, Number 550]

- Ibn 'Abbas said to me, "Shall I show you a woman of the people of Paradise?" I said, "Yes." He said, "This black lady came to the Prophet and said, 'I get attacks of epilepsy and my body becomes uncovered; please invoke Allah for me.' The Prophet said (to her), 'If you wish, be patient and you will (enter) Paradise; and if you wish, I will invoke Allah to cure you.' She said, 'I will remain patient,' and added, 'but I become uncovered, so please invoke Allah for me that I may not become uncovered.' So he invoked Allah for her." [Sahih Bukhari Volume 7, Book 70, Number 555]

-Narrated 'A'isha: Whenever Allah's Apostle paid a visit to a patient, or a patient was brought to him, he used to invoke Allah, saying, "Take away the disease, O the Lord of the people! Cure him as You are the One Who cures. There is no cure but Yours, a cure that leaves no disease." [Sahih Bukhari Volume 7, Book 70, Number 579]

(Info taken from a paper by Rabia Mills)