Musnad Ahmad 32:1459

Yunus narrated to us, Uthman ibn Rashid narrated to us, he said: Anas ibn Sirin narrated to me, he said: We came to Anas ibn Malik on a Thursday, and he called for his dining table, and he invited them to lunch. Some of the people ate lunch, and some refrained. Then they came to him on a Monday, and he did the same thing. He called for his dining table, then he invited them to lunch. Some of the people ate, and some refrained. Anas ibn Malik said to them: "Perhaps you are 'Thursday-observers,' perhaps you are 'Monday-observers.' The Messenger of Allah (peace and blessings of Allah be upon him) used to fast until he would not break it, until we would say: 'There is no intention in the soul of the Messenger of Allah (peace and blessings of Allah be upon him) to break his fast this year.' Then he would break his fast and not fast, until we would say: 'There is no intention in his soul to fast this year.' And the most beloved of fasting to him was in Sha'ban."