Sahih Muslim Book 7, Hadith Number 2994.

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 with him) reported: When Allah’s Messenger (may peace be upon him) had thrown pebbles at the Jamra and had sacrificed the animal, he turned (the right side) of his head towards the barber, and i.e. shaved it. He then called Abu Talha al-Ansari and gave it to him. He then turned his left side and asked him (the barber) to shave. And he (the barber) shaved and gave it to Abu Talha and told him to distribute it amongst the people.