Musnad Ahmad 1481

It was narrated from Mus"ab bin Sa"d that his father said: I said: “O Messenger of Allah, which people are most severely tested?” He said: "The prophets, then the righteous, 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 his test will be lighter. A slave [of Allah] will be tested until he walks upon the face of the earth with not a single sin on him."