It is narrated from Hazrat Anas (may Allah be pleased with him) that someone asked Hazrat Anas (may Allah be pleased with him) about the night prayer and voluntary fasts of the Prophet (peace and blessings be upon him). He replied that whenever we wished to see the Prophet (peace and blessings be upon him) praying at night, we could see him, and whenever we intended to see him sleeping, we could also see that. Similarly, in some months, the Prophet (peace and blessings be upon him) would fast so continuously that we would think he would not leave any fast, and at other times he would leave fasting to the extent that we would say perhaps now the Prophet (peace and blessings be upon him) will not fast anymore.