Sahih Al Bukhari Hadith Number 1971

Narrated Ibn `Abbas: The Prophet (peace be upon him) 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.”

30  Fasting
Hadith number 1971
Volume no 3
Chapter: 53 (Fasting and non-fasting (periods) of the Prophet saws.)