Sahih Al Bukhari Hadith Number 1141

Narrated Anas bin Malik: Sometimes Allah’s Messenger (peace be upon him) would not fast (for so many days) that we thought that he would not fast that month and he sometimes used to fast (for so many days) that we thought he would not leave fasting throughout that month and (as regards his prayer and sleep at night), if you wanted to see him praying at night, you could see him praying and if you wanted to see him sleeping, you could see him sleeping.

19  Prayer at Night (Tahajjud)
Hadith number 1141
Volume no 2
Chapter: 11 (The waking up of the Prophet (peace be upon him) from his sleep for the night prayer)