Ibn Hibban

Mus'ab bin Sa'd narrates from his father: I asked, "O Messenger of Allah (peace be upon him)! Who among the people are tested the most?" The Prophet (peace be upon him) said: "The Prophets, then those closest to them (in religious rank), then those closest to them. A person is tested according to the strength of his religion. If his religion is strong, his trial is made more severe, and if there is weakness in his religion, he is tested according to the level of his religion. And a person continues to be tested until he walks upon the earth with no sin left upon him (i.e., all his sins have been forgiven)."