Musnad Abu Ya'la al-Mawsili 6:142

Abu al-Rabi' al-Zahrani narrated to us, Hammad narrated to us, 'Asim narrated to us, from Mus'ab ibn Sa'd, from his father, who said: I said, "O Messenger of Allah, 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 servant is tested according to the measure of his religion." He said, "The trial does not cease to befall the servant until he walks upon the earth and there is no sin upon him." Hammad said: 'Asim shook it [his hand].