Mishkat al-Masabih 7 Fasting كتاب الصوم 114

وَعَنِ ابْنِ عَبَّاسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يُفْطِرُ أَيَّامَ الْبيض فِي حضر وَلَا فِي سفر. رَوَاهُ النَّسَائِيّ

Ibn ‘Abbas said that it was the custom of God’s messenger not to eat on the middle days* of the month whether travelling or not. *Literally, the days of the white (nights). These are the days following the thirteenth, fourteenth, and fifteenth nights of the month, the nights when there is most moonlight. Nasa’i transmitted it.