Sahih Muslim | Book 22 | Hadith Number 4873

Narrated by Umm Salama
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.