Musnad Ahmad 8:168

'Affan narrated to us, Hammad bin Zayd narrated to us, 'Asim bin Bahdalah narrated to us, Mus'ab bin Sa'd narrated to me, from his father, who said: I said to 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 who are most like them, then those who are most like them. A man is tested according to his religion; if his religion is firm, his trial is intensified, and if there is softness in his religion, he is tested according to his religion. The trial does not cease to afflict the servant until it leaves him walking upon the earth with no sin upon him."