Sahih Muslim Book 6, Hadith Number 2497.

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

Umm al-Fadl (Allah be pleased with her) is reported to have said that some people among the Companions of the Messenger of Allah (may peace be upon him) were in doubt about fasting on the day of ‘Arafa and we were with him on that day. I (Umm al-Fadl) sent him a cup of milk and he was halting at ‘Arafa, and he drank that.