Sunan Ibn Majah, Chapter: The Chapters on Tribulations

"I entered upon the Prophet (saw) when he was suffering from a fever, I placed my hand on him and felt heat with my hand from above the blanket. I said: 'O Messenger of Allah, how hard it is for you!' He said: 'We (Prophets) are like that. The trial is multiplied for us and so is the reward.' I said: 'O Messenger of Allah, which people are most severely tested?' He said: 'The Prophets.' I said: 'O Messenger of Allah, then who?' He said: 'Then the righteous, some of whom were tested with poverty until they could not find anything except a cloak to put around themselves. One of them will rejoice at calamity as one of you would rejoice at ease.'"