Volume 3 · Book 31 · Hadith 210
Narrated Maimuna: The people doubted whether the Prophet was fasting on the day of 'Arafat or not, so I sent milk while he was standing at 'Arafat, he drank it and the people were looking at him.
— Sahih Bukhari, Volume 3, Book 31: Fasting, Hadith #210