Sahih Muslim , Book 13, Hadith 227 (In-Book) ⓘ

'A'isha, the Mother of the Believers (Allah be pleased with her), reported that the Messenger of Allah used to fast (so continuously) that we said that he would not break, and did not fast at all till we said that he would not fast. And I did not see the Messenger of Allah completing the fast of a month, but that of Ramadan, and I did not see him fasting more in any other month than that of Sha'ban.

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِي النَّضْرِ، مَوْلَى عُمَرَ بْنِ عُبَيْدِ اللَّهِ عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ، - رضى الله عنها - أَنَّهَا قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ ‏.‏ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ ‏.‏ وَمَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم اسْتَكْمَلَ صِيَامَ شَهْرٍ قَطُّ إِلاَّ رَمَضَانَ وَمَا رَأَيْتُهُ فِي شَهْرٍ أَكْثَرَ مِنْهُ صِيَامًا فِي شَعْبَانَ ‏.‏