Musnad Ahmad 32:72

Ibn Abi ‘Adi narrated to us, from Humayd, who said: Anas was asked about the prayer of the Messenger of Allah (may Allah bless him and grant him peace) at night, and he said: "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. And he would fast during the month until we would say: 'He does not break his fast at all during it,' and he would break his fast until we would say: 'He does not fast at all during it.'"