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 ﷺ شَهْرًا كَامِلاً قَطُّ غَيْرَ رَمَضَانَ وَيَصُومُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يَصُومُ  

The Messenger of Allah ﷺ did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast.  

مَا صَامَ رَسُولُ اللَّهِ ﷺ شَهْرًا كَامِلاً قَطُّ غَيْرَ رَمَضَانَ وَكَانَ يَصُومُ إِذَا صَامَ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يُفْطِرُ وَيُفْطِرُ إِذَا أَفْطَرَ حَتَّى يَقُولَ الْقَائِلُ لاَ وَاللَّهِ لاَ يَصُومُ  

The Messenger of Allah ﷺ did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast. (Using translation from Muslim 1157a)  

وَاللهِ مَا صَامَ رَسُولُ اللهِ ﷺ شَهْرًا كَامِلًا قَطُّ غَيْرَ رَمَضَانَ وَكَانَ إِذَا صَامَ صَامَ حَتَّى يَقُولَ الْقَائِلُ لَا وَاللهِ لَا يُفْطِرُ وَيُفْطِرُ إِذَا أَفْطَرَ حَتَّى يَقُولَ الْقَائِلُ وَاللهِ لَا يَصُومُ  

The Messenger of Allah ﷺ did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast. (Using translation from Muslim 1157a)  

وَاللهِ مَا صَامَ رَسُولُ اللهِ ﷺ شَهْرًا كَامِلًا قَطُّ غَيْرَ رَمَضَانَ وَكَانَ إِذَا صَامَ صَامَ حَتَّى يَقُولَ الْقَائِلُ وَاللهِ لَا يُفْطِرُوَيُفْطِرُ إِذَا أَفْطَرَ حَتَّى يَقُولُ الْقَائِلُ وَاللهِ لَا يَصُومُ  

The Messenger of Allah ﷺ did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast. (Using translation from Muslim 1157a)  

مَا صَامَ النَّبِيُّ ﷺ شَهْرًا كَامِلًا غَيْرَ رَمَضَانَ وَإِنْ كَانَ لَيَصُومُ إِذَا صَامَ حَتَّى يَقُولَ الْقَائِلُ لَا وَاللَّهِ لَا يُفْطِرُ وَيُفْطِرُ إِذَا أَفْطَرَ حَتَّى يَقُولَ الْقَائِلُ لَا وَاللَّهِ لَا يَصُومُ  

The Messenger of Allah ﷺ did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast. (Using translation from Muslim 1157a)  

مَا صَامَ رَسُولُ اللهِ ﷺ شَهْرًا كَامِلًا غَيْرَ رَمَضَانَ وَكَانَ يَصُومُ إِذَا صَامَ حَتَّى يَقُولَ الْقَائِلُ لَا وَاللهِ لَا يُفْطِرُ وَيُفْطِرُ إِذَا أَفْطَرَ حَتَّى يَقُولَ الْقَائِلُ لَا وَاللهِ لَا يَصُومُ