Sahih Muslim 1123
Sahih Muslim · Book 13: The Book of Fasting · It is recommended for the person performing Hajj in 'Arafat not to fast on the day of 'Arafah.
وَحَدَّثَنِي هَارُونُ بْنُ سَعِيدٍ الأَيْلِيُّ ، حَدَّثَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي عَمْرٌو ، أَنَّ أَبَا النَّضْرِ ، حَدَّثَهُ أَنَّ عُمَيْرًا مَوْلَى ابْنِ عَبَّاسٍ - رضى الله عنهما - حَدَّثَهُ أَنَّهُ، سَمِعَ أُمَّ الْفَضْلِ ، - رضى الله عنها - تَقُولُ شَكَّ نَاسٌ مِنْ أَصْحَابِ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي صِيَامِ يَوْمِ عَرَفَةَ وَنَحْنُ بِهَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَأَرْسَلْتُ إِلَيْهِ بِقَعْبٍ فِيهِ لَبَنٌ وَهُوَ بِعَرَفَةَ فَشَرِبَهُ .
Umm al-Fadl (Allah be pleased with her) is reported to have said that some people among the Companions of the Messenger of Allah (ﷺ) were in doubt about fasting on the day of 'Arafa and we were with him on that day. I (Umm al-Fadl) sent him a cup of milk and he was halting at 'Arafa, and he drank that.