Sahih Muslim 1139
Sahih Muslim · Book 13: The Book of Fasting · The prohibition of fasting on the two days of 'Id
Ziyad b. Jubair reported that a person came to Ibn 'Umar (Allah be pleased with them) and said:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا وَكِيعٌ ، عَنِ ابْنِ عَوْنٍ ، عَنْ زِيَادِ بْنِ جُبَيْرٍ ، قَالَ جَاءَ رَجُلٌ إِلَى ابْنِ عُمَرَ - رضى الله عنهما - فَقَالَ إِنِّي نَذَرْتُ أَنْ أَصُومَ يَوْمًا فَوَافَقَ يَوْمَ أَضْحَى أَوْ فِطْرٍ . فَقَالَ ابْنُ عُمَرَ رضى الله عنهما أَمَرَ اللَّهُ تَعَالَى بِوَفَاءِ النَّذْرِ وَنَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ صَوْمِ هَذَا الْيَوْمِ .
I had taken a vow that I would fast on the day (but it accidentally) synchronises with the day of Adha or the day of Fitr. Thereupon Ibn 'Umar (Allah be pleased with him) said: Allah, the Exalted, has commanded fulfilling of the vow, but the Messenger of Allah (ﷺ) has forbidden the observance of fast on this day.