Sahih Muslim Book 7, Hadith Number 2993.

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 that Allah’s Messenger (may peace be upon him) threw stones at Jamrat al-‘Aqaba. He then want to his sacrificial animal and sacrificed it, and there was sitting the barber, and he pointed with his hand towards his head, and he shaved the right half of it, and he (the Holy Prophet) distributed them (the hair) among those who were near him. And he again said: Shave the other half, and said: Where is Abu Talha and gave it (the hair) to him.