Bukhari :: Book 3 :: Volume 31 :: Hadith 192

Narrated Ibn 'Abbas:

The Prophet never fasted a full month except the month of Ramadan, and he used to fast till one could say, "By Allah, he will never stop fasting," and he would abandon fasting till one would say, "By Allah, he will never fast."

Source materials are from the University of Southern California MSA site
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