first ten days of Dhu’l Hijja

عن أم سلمة أن النبي صلى الله عليه وسلم قال إذا رأيتم هلال ذي الحجة وأراد أحدكم أن يضحي فليمسك عن شعره وأظفاره

Umm Salama reported (these words) directly from Allah's Messenger (pbuh): If anyone has in his possession  a sacrificial animal to offer as a sacrifice (on E'Id al-Adha), he should not get his hair cut and nails trimmed during the first days of Dhu’l Hijja[1]

سمعت أم سلمة زوج النبي صلی الله عليه وسلم تقول قال رسول الله صلی الله عليه وسلم من کان له ذبح يذبحه فإذا أهل هلال ذي الحجة فلا يأخذن من شعره ولا من أظفاره شيا حتی يضحي

Allah's Messenger (pbuh) to have said: He who has a sacrificial animal with him whom (he intends) to offer as a sacrifice, and he enters the month of Dhu'l-Hijja, he should not get his hair cut or nails trimmed until he has sacrificed the animal. [2]

In the light of these hadith, if a person is going to offer sacrifice, he has ordered not to cut his nails, hair when days of Dahul Hajj enter. And it is haram for the person who is going to sacrifice to cut nail and hair.

Note hadiths no. are taken from Easy Quran and hadith.

1 Muslim book of sacrifice 12252                                                                                                   
2 Muslim book of sacrifice 12254