Musnad Ahmad 32:1529

Muhammad ibn Abdullah narrated to us, Humayd narrated to us, from Anas ibn Malik, that he was asked about the prayer of the Prophet, peace and blessings of Allah be upon him, and his voluntary fasting. He said: "He used to fast during the month until we would say, 'He does not intend to break any part of it,' and he would break his fast until we would say, 'He does not intend to fast any part of it.' And we would not wish to see him praying at night except that we saw him, and we would not wish to see him sleeping except that we saw him."