Sahih Bukhari, Chapter: Prayer at Night (Tahajjud)

Sometimes Allah's Apostle 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 through-out 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.