Sunan an-Nasai Book 22, Hadith 2185

أَخْبَرَنَا أَبُو الأَشْعَثِ، عَنْ يَزِيدَ، - وَهُوَ ابْنُ زُرَيْعٍ - قَالَ حَدَّثَنَا الْجُرَيْرِيُّ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ، قَالَ قُلْتُ لِعَائِشَةَ أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي صَلاَةَ الضُّحَى قَالَتْ لاَ إِلاَّ أَنْ يَجِيءَ مِنْ مَغِيبِهِ ‏.‏ قُلْتُ هَلْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم لَهُ صَوْمٌ مَعْلُومٌ سِوَى رَمَضَانَ قَالَتْ وَاللَّهِ إِنْ صَامَ شَهْرًا مَعْلُومًا سِوَى رَمَضَانَ حَتَّى مَضَى لِوَجْهِهِ وَلاَ أَفْطَرَ حَتَّى يَصُومَ مِنْهُ ‏.‏

It was narrated that 'Abdullah bin Shaqiq said:'I said to Aishah: 'did the Messenger of Allah offer Duah prayer?' she said: 'No, unless he was returning from a journey.' I said: 'was the Messenger of Allah known to observe any fast regularly apart from Ramadan?'' She said: 'by Allah, he did not observe any fast regularly apart from amadan until he passed away, and he did not break his fast for a whole month, rather he would fast some of it (each month)