Sahih Muslim Book 7, Hadith Number 2992.

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.

Abu Bakr reported: (He called for) the barber and, pointing towards the right side of his head, said: (Start from) here, and then distributed his hair among those who were near him. He then pointed to the barber (to shave) the left side and he shaved it, and he gave (these hair) to Umm Sulaim (Allah be pleased with her). And in the narration of Abu Kuraib (the words are): “He started from the right half (of his head), and he distributed a hair or two among the people and then (asked the barber) to shave the left side and he did similarly, and he (the Holy Prophet) said: Here is Abu Talha and he gave these (hair) to Abu Talha.”