The Book of Sacrifices
35/7. 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
Sahih Muslim
وَحَدَّثَنَاهُ إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا سُفْيَانُ، حَدَّثَنِي عَبْدُ الرَّحْمَنِ بْنُ حُمَيْدِ بْنِ، عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أُمِّ سَلَمَةَ، تَرْفَعُهُ قَالَ " إِذَا دَخَلَ الْعَشْرُ وَعِنْدَهُ أُضْحِيَّةٌ يُرِيدُ أَنْ يُضَحِّيَ فَلاَ يَأْخُذَنَّ شَعْرًا وَلاَ يَقْلِمَنَّ ظُفُرًا " .
Umm Salama reported Allah's Apostle (ﷺ) as saying: If anyone of you intends to offer sacrifice he should not get his hair cut or nails trimmed.
Reference : Sahih Muslim 1977dIn-book reference : Book 35, Hadith 54USC-MSA web (English) reference : Book 22, Hadith 4870