Sahih Muslim | Book 22 | Hadith Number 4871

Narrated by Umm Salama
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