Sahih Muslim Book 22, Hadith Number 4870.

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 reported Allah’s Apostle (may peace be upon him) as saying: If anyone of you intends to offer sacrifice he should not get his hair cut or nails trimmed.