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
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. 
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