Sahih Muslim 1157a

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."

مَا صَامَ النَّبِيُّ صلى الله عليه وسلم شَهْرًا كَامِلاً قَطُّ غَيْرَ رَمَضَانَ، وَيَصُومُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يُفْطِرُ، وَيُفْطِرُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يَصُومُ‏.‏

Narrated by Ibn ‘Abbās

" During any month continuously since he came to Medina."

وَحَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ وَأَبُو بَكْرِ بْنُ نَافِعٍ عَنْ غُنْدَرٍ عَنْ شُعْبَةَ عَنْ أَبِي بِشْرٍ بِهَذَا الْإِسْنَادِ وَقَالَ شَهْرًا مُتَتَابِعًا مُنْذُ قَدِمَ الْمَدِينَةَ

I said to 'A'isha (Allah be pleased with her): Did the Messenger of Allah (ﷺ) observe fast during a month? She said, I do not know of any month in which he fasted throughout except Ramadan and (the month) in which he did not fast at all till he ran the course of his life. May peace be upon him.

مَا عَلِمْتُهُ صَامَ شَهْرًا كُلَّهُ إِلاَّ رَمَضَانَ وَلاَ أَفْطَرَهُ كُلَّهُ حَتَّى يَصُومَ مِنْهُ حَتَّى مَضَى لِسَبِيلِهِ صلى الله عليه وسلم ‏.‏

Narrated by Ā’ishah + 17 other versions