Musannaf Ibn Abi Shayba 33:123

Yazid ibn Harun narrated to us, he said: Hisham informed us, from al-Hasan, who said: The Prophet, may Allah bless him and grant him peace, would be overcome by worship until he would come out to the people as if he were a worn-out waterskin, and he was the most fasting of people. It was said: "O Messenger of Allah, has Allah not forgiven you?" He said: "Shall I not be a grateful servant?"