If somebody has vowed that he will observe Saum

Narrated `Abdullah bin `Umar:

that he was asked about a man who had vowed that he would fast all the days of his life then the day of `Id al Adha or `Id-al-Fitr came. `Abdullah bin `Umar said: You have indeed a good example in Allah's Messenger (ﷺ) . He did not fast on the day of `Id al Adha or the day of `Id-al-Fitr, and we do not intend fasting on these two days.

حَدَّثَنَا مُحَمَّدُ بْنُ أَبِي بَكْرٍ الْمُقَدَّمِيُّ، حَدَّثَنَا فُضَيْلُ بْنُ سُلَيْمَانَ، حَدَّثَنَا مُوسَى بْنُ عُقْبَةَ، حَدَّثَنَا حَكِيمُ بْنُ أَبِي حُرَّةَ الأَسْلَمِيُّ، أَنَّهُ سَمِعَ عَبْدَ اللَّهِ بْنَ عُمَرَ ـ رضى الله عنهما ـ سُئِلَ عَنْ رَجُلٍ، نَذَرَ أَنْ لاَ، يَأْتِيَ عَلَيْهِ يَوْمٌ إِلاَّ صَامَ، فَوَافَقَ يَوْمَ أَضْحًى أَوْ فِطْرٍ‏.‏ فَقَالَ لَقَدْ كَانَ لَكُمْ فِي رَسُولِ اللَّهِ أُسْوَةٌ حَسَنَةٌ، لَمْ يَكُنْ يَصُومُ يَوْمَ الأَضْحَى وَالْفِطْرِ، وَلاَ يَرَى صِيَامَهُمَا‏.‏

Narrated Ziyad bin Jubair:

I was with Ibn `Umar when a man asked him, "I have vowed to fast every Tuesday or Wednesday throughout my life and if the day of my fasting coincided with the day of Nahr (the first day of `Id-al- Adha), (What shall I do?)" Ibn `Umar said, "Allah has ordered the vows to be fulfilled, and we are forbidden to fast on the day of Nahr." The man repeated his question and Ibn `Umar repeated his former answer, adding nothing more.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، عَنْ يُونُسَ، عَنْ زِيَادِ بْنِ جُبَيْرٍ، قَالَ كُنْتُ مَعَ ابْنِ عُمَرَ فَسَأَلَهُ رَجُلٌ فَقَالَ نَذَرْتُ أَنْ أَصُومَ كُلَّ يَوْمِ ثَلاَثَاءَ أَوْ أَرْبِعَاءَ مَا عِشْتُ، فَوَافَقْتُ هَذَا الْيَوْمَ يَوْمَ النَّحْرِ‏.‏ فَقَالَ أَمَرَ اللَّهُ بِوَفَاءِ النَّذْرِ، وَنُهِينَا أَنْ نَصُومَ يَوْمَ النَّحْرِ‏.‏ فَأَعَادَ عَلَيْهِ فَقَالَ مِثْلَهُ، لاَ يَزِيدُ عَلَيْهِ‏.‏