(7) باب نَهْيِ مَنْ دَخَلَ عَلَيْهِ عَشْرُ ذِي الْحِجَّةِ وَهُوَ مُرِيدُ التَّضْحِيَةِ أَنْ يَأْخُذَ مِنْ شَعْرِهِ أَوْ أَظْفَارِهِ شَيْئًا
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 (ES)
Reference (ES): Sahih Muslim 1977 d (ES)
In-book reference (ES): Book 35, Hadith 55 (ES)
USC-MSA web (English) reference(deprecated numbering scheme): Book 22, Hadith 4871 (ES)