Umm al-Fadl bint- al-Harith reported that some people argued about the fasting of the Messenger of Allah (ﷺ) on the day of 'Arafa. Some of them said that he had been fasting, whereas the others said that he had not been fasting. I sent a cup of milk to him while he was riding his camel at 'Arafa, and he drank it.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِي النَّضْرِ، عَنْ عُمَيْرٍ، مَوْلَى عَبْدِ اللَّهِ بْنِ عَبَّاسٍ عَنْ أُمِّ الْفَضْلِ بِنْتِ الْحَارِثِ، أَنَّ نَاسًا، تَمَارَوْا عِنْدَهَا يَوْمَ عَرَفَةَ فِي صِيَامِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ بَعْضُهُمْ هُوَ صَائِمٌ وَقَالَ بَعْضُهُمْ لَيْسَ بِصَائِمٍ . فَأَرْسَلْتُ إِلَيْهِ بِقَدَحِ لَبَنٍ وَهُوَ وَاقِفٌ عَلَى بَعِيرِهِ بِعَرَفَةَ فَشَرِبَهُ .
sunnah.com