Sahih Muslim Book 7, Hadith Number 2939.

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

Muhammad b. Abu Bakr al-Thaqafi asked Anas b. Malik (Allah be pleased with him), while on their way from Mina to ‘Arafa in the morning: What did you do on this day in the company of Allah’s Messenger (may peace be upon him)? Thereupon he said: One of us pronounced Tahlil, and he met with no disapproval, and one of us pronounced Takbir, and he also met with no disapproval.