If anyone of you intends to offer sacrifice he should not get his hair cut or nails trimmed.
Sahih Muslim Book 35, Hadith 54
← Previous Hadith 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).

Sahih Muslim Book 35, Hadith 53 Next Hadith → 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

Sahih Muslim Book 35, Hadith 55