Sahih Bukhari Volume 003, Book 031, Hadith Number 193.

Narated By Anas : Allah's Apostle used to leave fasting in a certain month till we thought that he would not fast in that month, and he used to fast in another month till we thought he would not stop fasting at all in that month. And if one wanted to see him praying at night, one could see him (in that condition), and if one wanted to see him sleeping at night, one could see him (in that condition) too.