Sunan an-Nasa'i, Book 48, Hadith 14 (In-Book) ⓘ
It was narrated that Anas said: "The hair of the Prophet [SAW] was wavy, neither curly nor straight, and (hung down) between his ears and his shoulders."
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا وَهْبُ بْنُ جَرِيرٍ، قَالَ حَدَّثَنَا أَبِي قَالَ، سَمِعْتُ قَتَادَةَ، يُحَدِّثُ عَنْ أَنَسٍ، قَالَ كَانَ شَعْرُ النَّبِيِّ صلى الله عليه وسلم شَعْرًا رَجِلاً لَيْسَ بِالْجَعْدِ وَلاَ بِالسَّبْطِ بَيْنَ أُذُنَيْهِ وَعَاتِقِهِ .
also referenced under: Hadith 5053 (Reference) & Volume 6, Book 48, Hadith 5056 (DarusSalam)