Anas (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) used to leave fasting in a certain month until we would think that he has not fasted at all that month; and he used to fast in another month so much that we would think he has not stopped fasting at all in that month. If one wished to see him praying at night, one could see him (in that state), and if one wished to see him sleeping at night, one could see him (in that state) too.
شرح الحديث :
The meaning of the Hadīth: Anas (may Allah be pleased with him) informs about the condition of the Prophet (may Allah's peace and blessings be upon him) regarding fasting and praying at night. He said that he used to leave fasting in a certain month until a person would think that he has not fasted any day of that month; for not fasting for many days. Conversely, he used to fast in a certain month so many days that one would think that he has not broken his fast at all in that month; for his consistent fasting for many days. Likewise, he has not committed himself to pray at a certain time at night. Rather, he used to pray at the beginning of the night sometimes, other times in the middle, and other at the last part of it; such that if you wished to see him praying at night, you would be able to. And if you wished to see him sleeping at night, you would be able to as well. Therefore, his deeds were moderate between excess and negligence.