The Messenger of Allah ﷺ used to fast to such an extent that we thought that he would never break his fast, and he would go without fasting to such an extent that we thought he would never fast. I never saw the Messenger of Allah ﷺ fast a complete month except in Ramadan, and I never saw his fast more in any month than in Sha'ban.  

عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ وَمَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ اسْتَكْمَلَ صِيَامَ شَهْرٍ قَطُّ إِلاَّ رَمَضَانَ وَمَا رَأَيْتُهُ فِي شَهْرٍ أَكْثَرَ صِيَامًا مِنْهُ فِي شَعْبَانَ  

He would fast all but a little of Sha'ban, rather he used to fast the whole of Sha'ban.  

عَنِ النَّبِيِّ ﷺ بِمَعْنَاهُ زَادَ كَانَ يَصُومُهُ إِلاَّ قَلِيلاً بَلْ كَانَ يَصُومُهُ كُلَّهُ