Musnad Ahmad 8:43

Waki’ narrated to us, Sufyan narrated to us, from ‘Asim ibn Abi al-Najud, 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 the righteous, then the next best, and the next best of the people. A man is tested according to his religion; if there is firmness in his religion, his trial is increased, and if there is thinness in his religion, it is lightened for him. And the trial continues to befall the servant until he walks upon the face of the earth with no sin upon him."