Hadith: Whoever has a sacrificial animal for slaughtering, once the crescent of Dhu al-Hijjah is declared, he should not cut anything of his hair or his nails until he sacrifices

عَن أُمِّ سَلَمَةَ زَوْجَ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ رضي الله عنها قَالت: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:

«مَنْ كَانَ لَهُ ذِبْحٌ يَذْبَحُهُ فَإِذَا أُهِلَّ هِلَالُ ذِي الْحِجَّةِ، فَلَا يَأْخُذَنَّ مِنْ شَعْرِهِ، وَلَا مِنْ أَظْفَارِهِ شَيْئًا حَتَّى يُضَحِّيَ».
[صحيح] - [رواه مسلم]

Umm Salamah (may Allah be pleased with her) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "Whoever has a sacrificial animal for slaughtering, once the crescent of Dhu al-Hijjah is declared, he should not cut anything of his hair or his nails until he sacrifices (the animal)."
Sahih/Authentic. - [Muslim]

Explanation

Umm Salamah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) forbade the one who intends to sacrifice an animal (in Eid al-Ad'ha) from cutting anything from his hair and nails until after his sacrificial animal is slaughtered. This means that when the first ten days of Dhu al-Hijjah start, and you wish to sacrifice an animal on your behalf, or on behalf of someone else, which you bought with your own money, then do not remove anything from your hair. This includes armpit hair, pubic hair, mustache, or the hair on the head. Also, do not clip your fingernails or toenails. This continues until the animal is slaughtered. Another narration by Imām Muslim reads: “... he should not remove anything from his hair or skin." This is done out of respect to the act of sacrificing, and in order for those who did not perform the Hajj or ‘Umrah to participate in some of the rituals performed by those in the state of Ihrām, since the latter are not allowed to remove any part of their hair or nails.