by Imam Muslim, translation by Abdul Hamid Siddiqui , Volume: The Book of Sacrifices (Kitab Al-Adahi)
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 DHU'L-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).
Umm Salama reported Allah's Apostle (may peace be upon him) as saying: If anyone of you intends to offer sacrifice he should not get his hair cut or nails trimmed.
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
This hadith has been narrated on the authority of 'Amr b. Muslim with the same chain of transmitters.
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.