The Book of Pilgrimage
15/55. Chapter:
Shaving the head is preferrable to cutting the hair, although cutting the hair is permissible
Sahih Muslim
وَحَدَّثَنَاهُ ابْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الْوَهَّابِ، حَدَّثَنَا عُبَيْدُ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ فِي الْحَدِيثِ فَلَمَّا كَانَتِ الرَّابِعَةُ قَالَ " وَالْمُقَصِّرِينَ " .
Ubaidullah reported this hadith with the same chain of transmitters and (it is said) that it was on the fourth turn that he (the Holy Prophet) said: (May Allah have mercy upon) those who got their hair clipped."
Reference : Sahih Muslim 1301dIn-book reference : Book 15, Hadith 351USC-MSA web (English) reference : Book 7, Hadith 2986