Shama-il Tirmidhi Chapter 003, Hadith Number 004 (026).

Qataadah bin Da'aamah As-Sadusi relates: "I asked Anas R.A., 'How was the hair of Rasulullah (SAW)?'. He replied: 'It was not very twisted, nor very straight. It had a slight twist and was a bit curled, and reached till his ear-lobes'".