Sahih Muslim Book 6, Hadith Number 2498.
Chapter : It is preferable for the pilgrim not to observe fast in ‘Arafat on the day of ‘Arafa.
Kuraib, the freed slave of Ibn ‘Abbas (Allah be pleased with him), reported from Maimuna, the wife of the Apostle of Allah (may peace be upon him), that people had doubt about the fasting of Allah’s Messenger (may peace be upon him) on the day of ‘Arafa. Maimuna sent him a cup of milk and he was halting at a place and he drank it and the people were seeing him.