Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5773
Qatada reported: I asked Anas b. Malik: How was the hair of Allah's Messenger (may peace be upon him)? Thereupon he said: His hair was neither very curly nor very straight, and they hung over his shoulders and earlobes.