Sahih Muslim Book - 35, Hadith - 5118

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.

It was narrated from Umm Salamah, attributing it to the Prophet (s.a.w): "When the first ten days begin, if he has a sacrificial animal that he wants to offer as a sacrifice, let him not remove anything from his hair or trim his nails."