From Sahih Bukhari (Hadith 5905):
Narrated Qatada:l asked Anas bin Malik about the hair of Allah's Messenger (ﷺ). He said, "The hair of Allah's Messenger (ﷺ) was neither much straight, nor much curly, and it used to hang down till between his shoulders and his earlobes
Reference: Bukhari 5905