Musnad Ahmad 8:116

Isma'il—meaning Ibn Ibrahim—narrated to us, Hisham al-Dastawa'i informed us, from 'Asim ibn Bahdalah, from Mus'ab ibn Sa'd, who said: Sa'd said: "O Messenger of Allah, which of the people are most severely tested?" He said: "The prophets, then the most righteous, then the most righteous. A servant is tested according to the measure of his religion. If his religion is firm, he is tested according to that measure"—and he said once: "most severely tested"—"and if there is softness in his religion, he is tested according to that measure"—and he said once: "according to the extent of his religion." He said: "The trials do not cease to afflict the servant until he walks upon the earth—meaning—while there is no sin upon him." My father said: And he said once: From Sa'd, he said: I said: "O Messenger of Allah."