Sahih Muslim Book 7, Hadith Number 2991.

Chapter : It is a sunnah that on the day of Nahar one should throw pebbles, then slaughter animal, then get one’s head shaved, and one should start shaving one’s head from the right side.

Anas b. Malik (Allah be pleased wish him) reported that Allah’s Messenger (may peace be upon him) came to Mina; he went to the Jamra and threw pebbles at it, after which he went to his lodging in Mina, and sacrificed the animal. He then called for a barber and, turning his right side to him, let him shave him; after which he timed his left side. He then gave (these hair) to the people.