Sahih Muslim Book 22, Hadith Number 4873.

Chapter : It is not permissible for one who intends to sacrifice the animal to get one’s hair or nails cut after the beginning of Dhull-Hijja.

Umm Salama, the wife of Allah’s Apostle (may peace be upon him), reported Allah’s Messenger (may peace be upon him) to have said: He who has a sacrificial animal with him whom (he intends) to offer as sacrifice, and he enters the month of Dhu’I-Hijja, he should not get his hair cut or nails trimmed until he has sacrificed the animal.