Where did evil come from?

All events in the universe occur by the decree and preordainment of Allah, as stated in His verses: "Indeed, We created all things in due measure" and "No disaster strikes except by permission of Allah." All of Allah's actions are good, as mentioned in the hadith: "All good is in Your hands, and evil is not attributed to You." What is preordained might appear as evil to a servant, such as illness, but it may contain hidden good, like expiation of sins and elevation of ranks, as in the hadith: "No fatigue, nor disease... befalls a Muslim but Allah expiates with it some of his sins." The believer's entire affair is good; if prosperity befalls him, he is grateful, and if adversity befalls him, he is patient. Calamity may also serve as a reminder for people to repent to Allah, as He says: "Corruption has appeared on land and sea by [reason of] what the hands of people have earned so He may let them taste part of [the consequence of] what they have done that perhaps they will return [to righteousness]," and establishing legal punishments expiates sins.