Sahih al-Bukhari 1141

It is related that Anas said, "The Messenger of Allah, may Allah bless him and grant him peace, used sometimes not to fast for so much of a month that we thought that he would not fast for any of it, and other times he would fast so much that we thought that he would not break his fast at all. If you wanted to see him doing nothing but praying all night, you could see him doing that, or doing nothing but sleeping, you could see him doing that as well." Sulayman and Abu Khalid al-Ahmar corroborated it from Humayd.