Allah’s Messenger (saws) said: ‘When any one of you intending to sacrifice the animal enters in the month (of Dhul-Hijjah) he should not get his hair or nails touched (cut).
Sahih Muslim Hadith 4869
Narrated by: Umm Salamah
“When you see the new moon of Dhul Hijjah, and one of you desires that he offer a sacrifice, then let him keep [i.e. not cut] his hair and nails.”
Sahih Muslim v. 13, p. 2452