Sahih Muslim 1301
Sahih Muslim · Book 15: The Book of Pilgrimage · Shaving the head is preferrable to cutting the hair, although cutting the hair is permissible
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:
وَحَدَّثَنَاهُ ابْنُ الْمُثَنَّى ، حَدَّثَنَا عَبْدُ الْوَهَّابِ ، حَدَّثَنَا عُبَيْدُ اللَّهِ ، بِهَذَا الإِسْنَادِ وَقَالَ فِي الْحَدِيثِ فَلَمَّا كَانَتِ الرَّابِعَةُ قَالَ " وَالْمُقَصِّرِينَ " .
(May Allah have mercy upon) those who got their hair clipped."