كتاب الحج
15  
The Book of Pilgrimage
(33)
Chapter: It is permissible for the pilgrim performing Umrah to shorten his hair and he does not have to shave it. It is recommended for him to shave his hair or shorten it at Al-Marwah
(33)
باب التَّقْصِيرِ فِي الْعُمْرَةِ ‏
Sahih Muslim 1246 a

Ibn Abbas reported that Mu'awiya had said to them:

Do you know that I clipped some hair from the head of Allah's Messenger (ﷺ) at al- Marwa with the help of a clipper? I said: I do not know it except as it verdict against you.
حَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ هِشَامِ بْنِ حُجَيْرٍ، عَنْ طَاوُسٍ، قَالَ قَالَ ابْنُ عَبَّاسٍ قَالَ لِي مُعَاوِيَةُ أَعَلِمْتَ أَنِّي قَصَّرْتُ مِنْ رَأْسِ رَسُولِ اللَّهِ صلى الله عليه وسلم عِنْدَ الْمَرْوَةِ بِمِشْقَصٍ فَقُلْتُ لَهُ لاَ أَعْلَمُ هَذَا إِلاَّ حُجَّةً عَلَيْكَ ‏.‏
Reference : Sahih Muslim 1246a
In-book reference : Book 15, Hadith 229
USC-MSA web (English) reference : Book 7, Hadith 2870
  (deprecated numbering scheme)