Sahih Muslim Book 7, Hadith Number 2938.

Chapter : Pronouncing of Talbiya and takbir while going from Mina to ‘Arafat on the day of ‘Arafa.

Abdullah b. ‘Umar reported on the authority of his father (Allah be pleased with them): We were along with Allah’s Messenger (way peace he upon him) in the morning of ‘Arafa (9th of Dhu’l-Hijja). Some of us pronounced Takbir and some of us Tahlil La ilaha ill-Allah). And to those of us who pronounced Takbir, I said: By Allah, how strange it is that you did not care to ask him: What did you see Allah’s Messenger (may peace be upon him) doing (on this occasion)?