Umm al-Fadl, daughter of al-Harith, said: On the day of 'Arafah some people near her argued whether the Messenger of Allah (ﷺ) was fasting, some saying that he was, and others saying that he was not. I, therefore, sent him a cup of milk while he was observing the halt at 'Arafah on his camel, and he drank it.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ أَبِي النَّضْرِ، عَنْ عُمَيْرٍ، مَوْلَى عَبْدِ اللَّهِ بْنِ عَبَّاسٍ عَنْ أُمِّ الْفَضْلِ بِنْتِ الْحَارِثِ، أَنَّ نَاسًا، تَمَارَوْا عِنْدَهَا يَوْمَ عَرَفَةَ فِي صَوْمِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ بَعْضُهُمْ هُوَ صَائِمٌ . وَقَالَ بَعْضُهُمْ لَيْسَ بِصَائِمٍ . فَأَرْسَلَتْ إِلَيْهِ بِقَدَحِ لَبَنٍ وَهُوَ وَاقِفٌ عَلَى بَعِيرِهِ بِعَرَفَةَ فَشَرِبَ .
sunnah.com