Umm Salama, the wife of Allah's Apostle (ﷺ), reported Allah's Messenger (ﷺ) 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.
Sahih Muslim · The Book of Sacrifices