A man went to Ibn ʿUmar I. and said, "A man vowed to fast one day (the sub-narrator thinks that he said that the day was Monday), and that day happened to be ʿId day." Ibn ʿUmar said, "Allah orders vows to be fulfilled and the Prophet ﷺ forbade the fasting on this day (i.e. Id).
جَاءَ رَجُلٌ إِلَى ابْنِ عُمَرَ ؓ فَقَالَ رَجُلٌ نَذَرَ أَنْ يَصُومَ يَوْمًا قَالَ أَظُنُّهُ قَالَ الاِثْنَيْنِ فَوَافَقَ يَوْمَ عِيدٍ فَقَالَ ابْنُ عُمَرَ أَمَرَ اللَّهُ بِوَفَاءِ النَّذْرِ وَنَهَى النَّبِيُّ ﷺ عَنْ صَوْمِ هَذَا الْيَوْمِ