Musnad Ahmed: Hadith 1555

Sa"d said: O Messenger of Allah, which of the people are most severely tested? He said: “The Prophets (ﷺ), then the next best, then the next best. And a person will be tested according to the level of his religious commitment. If his religious commitment is solid, he will be tested accordingly - and on one occasion he said: his test will be more severe - and if there is some weakness in his religious commitment, he will be tested accordingly - and on one occasion he said: according to his level of religious commitment -. And trials will never disappear from (the life of) a person until he walks on the earth with not even one sin on him."