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

Umm Salama reported Allah's Messenger (ﷺ) having said this:

When any one of you intending to sacrifice the animal enters in the month (of Dhu'l-Hijja) he should not get his hair or nails touched (cut). It was said to Sufyan that some of the (scholars) did not deem this hadith to be Maffu'. He said: But I deem it as Marfu' (i. e. chain of narration traceable right up to the Holy Prophet).
حَدَّثَنَا ابْنُ أَبِي عُمَرَ الْمَكِّيُّ، حَدَّثَنَا سُفْيَانُ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، بْنِ عَوْفٍ سَمِعَ سَعِيدَ بْنَ الْمُسَيَّبِ، يُحَدِّثُ عَنْ أُمِّ سَلَمَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا دَخَلَتِ الْعَشْرُ وَأَرَادَ أَحَدُكُمْ أَنْ يُضَحِّيَ فَلاَ يَمَسَّ مِنْ شَعَرِهِ وَبَشَرِهِ شَيْئًا ‏"‏ ‏.‏ قِيلَ لِسُفْيَانَ فَإِنَّ بَعْضَهُمْ لاَ يَرْفَعُهُ قَالَ لَكِنِّي أَرْفَعُهُ ‏.‏
Reference : Sahih Muslim 1977 c
In-book reference : Book 35, Hadith 53
USC-MSA web (English) reference : Book 22, Hadith 4869
  (deprecated numbering scheme)