Musnad Ahmad 8:56

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Asim ibn Bahdalah, who said: I heard Mus'ab ibn Sa'd narrating from Sa'd, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him: "Which of the people are most severely tested?" He said: "The prophets, then those most like them, then those most like them. A man is tested according to his religion; if his religion is firm, his test is increased accordingly, and if his religion is weak, he is tested according to that." He said: "The trials will continue to afflict a man until he walks upon the earth with no sin upon him."