Sahih Muslim Book 6, Hadith Number 2494.

Chapter : It is preferable for the pilgrim not to observe fast in ‘Arafat on the day of ‘Arafa.

Umm al-Fadl bint-al-Harith reported that some people argued about the fasting of the Messenger of Allah (may peace be upon him) 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.