Sahih Muslim Book 22, Hadith Number 4871.

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 (these words) directly from Allah’s Messenger (may peace be upon him): If anyone has in his possession a sacrificial animal to offer as a sacrifice (on ‘Id al-Adha), he should not get his hair cut and nails trimmed after he has entered the first days of Dhu’l Hijja