Musnad Ahmad 1606

It was narrated from Mus"ab bin Sa"d, that his father said: I said to the Messenger of Allah (ﷺ). Which people are most severely tested? He said: “The Prophet (ﷺ)s, then the next best and the next best. A man will be tested according to his level of religious commitment: if his religious commitment is solid, his test will be greater and if there is some weakness in his religious commitment he will be tested in accordance with his level of religious commitment. Tests will continue to befall a slave [of Allah] until he walks upon the face of the earth with not a single sin on him."