Some people were disputing in her presence on the Day of Arafah about whether the Messenger of Allah ﷺ was fasting. Some said, "He is fasting." Others said, "He is not fasting." So she sent him a cup of milk while he was standing on his camel at Arafah, and he drank it.
أن ناسا تماروا عندها يوم عرفة في صيام رسول الله ﷺ، فقال بعضهم: هو صائم. وقال بعضهم: ليس بصائم، فأرسلت إليه بقدح لبن وهو واقف على بعيره بعرفة، فشربه.
Malik narrated it in Hajj (132) from Abu Nadr, from Umayr, the freed slave of Abdullah bin Abbas, from Umm Fadl, and related it as mentioned. Bukhari narrated it in Hajj (1661), and Muslim in Fasting (1123), both through the route of Malik with the same wording. Umm Fadl is the wife of Abbas bin Abdul-Muttalib. Her name is Lubaba bint Harith Hilaliyya.Ibn Khuzayma narrated it (2102) through the route of Ayub, from Ikrima, from Ibn Abbas, from his mother Umm Fadl, that the Messenger of Allah ﷺ broke his fast at Arafah, milk was brought to him, and he drank it.His saying, "tamaru," means they disagreed and argued. The reason for this was a conflict between the Prophet's ﷺ encouragement to fast on the day of Arafah, and the fact that they were occupied with the rites of Hajj and were on a journey. So the dispute arose, and when the Prophet ﷺ drank the milk, they understood that the merit of fasting on the day of Arafah applies to someone other than the pilgrim, and that it is preferable to forgo fasting on the day of Arafah while actually at Arafah.