كتاب الأضاحى
35
The Book of Sacrifices
(7)
Chapter: When the first ten days of Dhul-Hijjah begin, it is forbidden for the one who wants to offer a sacrifice to remove anything from his hair, nails or skin
(7)
باب نَهْيِ مَنْ دَخَلَ عَلَيْهِ عَشْرُ ذِي الْحِجَّةِ وَهُوَ مُرِيدُ التَّضْحِيَةِ أَنْ يَأْخُذَ مِنْ شَعْرِهِ أَوْ أَظْفَارِهِ شَيْئًا
Sahih Muslim 1977 d

Umm Salama reported (these words) directly from Allah's Messenger (ﷺ):

If anyone has in his possession a sacrificial animal to offer as a sacrifice (on 'Id al-Adha), he should not get his hair cut and nails trimmed after he has entered the first days of Dhu'l Hijja
وَحَدَّثَنِي حَجَّاجُ بْنُ الشَّاعِرِ، حَدَّثَنِي يَحْيَى بْنُ كَثِيرٍ الْعَنْبَرِيُّ أَبُو غَسَّانَ، حَدَّثَنَا شُعْبَةُ، عَنْ مَالِكِ بْنِ أَنَسٍ، عَنْ عُمَرَ بْنِ مُسْلِمٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أُمِّ سَلَمَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا رَأَيْتُمْ هِلاَلَ ذِي الْحِجَّةِ وَأَرَادَ أَحَدُكُمْ أَنْ يُضَحِّيَ فَلْيُمْسِكْ عَنْ شَعْرِهِ وَأَظْفَارِهِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1977 d
In-book reference : Book 35, Hadith 55
USC-MSA web (English) reference : Book 22, Hadith 4871
  (deprecated numbering scheme)