Sahih Muslim Book 22, Hadith Number 4869.

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 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). It was said to Sufyan that some of the (scholars) did not deem this hadith to be Maffu’. He said: But I deem it as Marfu’ (i.e. chain of narration traceable right up to the Holy Prophet).