Shama-il Tirmidhi Chapter 003, Hadith Number 007 (029).
Ibne Abbas R.A. says: "Rasulullah (SAW) used to leave his hair the way it naturally was, without making a path in the hair (parting hair). The reason being that the mushrikeen (polytheists) used to make a path in their hair, and the Ahlul Kitaab (People of the Book) did not do so. In the early periods Rasulullah (SAW) preferred to follow the Ahlul Kitaab, rather than others, in matters where no command had came from Allah. Later this was abrogated, and Rasulullah (SAW) began apposing the ways of the Ahlul Kitaab after this".