(53) باب مَا يُذْكَرُ مِنْ صَوْمِ النَّبِيِّ صلى الله عليه وسلم وَإِفْطَارِهِ
Narrated Ibn `Abbas:
The Prophet (ﷺ) 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."
Reference: Sahih al-Bukhari 1971
In-book reference: Book 30, Hadith 78
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 192