Umm al-Fadl bint al-Harith (may Allah be pleased with her) narrates that some people disputed in her presence on the Day of Arafah about the Messenger of Allah (peace and blessings be upon him). Some of them said: He is fasting, and some of them said: He is not fasting. So Umm al-Fadl (may Allah be pleased with her) sent him a cup of milk while he was standing at Arafah on a camel, and he drank it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8384