بسم الله الرحمن الرحيم

Umm Salama Radi allahu anha se rivayat hai ki Allah ke Rasooll Sallallahu alaihi wa sallam ne farmaya jab tum Zil Hijja (Dhul Hijja) ka chand dekho aur tum mein se koi Qurbani karna chahe to apne baal aur nakhun yuhi rahne de (yani na kaate)
[Sahih Muslim , Vol 5, #5119]

English Translation:
Umm Salama Radi allahu anha reported Allah’s Messenger may peace be upon him having said this: When any one of you intending to sacrifice the animal enters in the month (of Dhu’l-Hijja) he should not get his hair or nails touched (cut).
[Sahih Muslim, Book 022, Number 4869]