It was narrated from Umm Salamah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said:

When you see the new moon of Dhu’l-Hijjah, and one of you wants to offer a sacrifice, let him refrain from (removing anything) from his hair or nails. (Narrated by Muslim)

According to another version:

Let him not remove anything of his hair or skin.

According to a version narrated by Abu Dawood, Muslim and al-Nasaai:

Whoever has a sacrifice to offer, when the new moon of Dhu’l-Hijjah begins, let him not remove anything from his hair or nails until he has offered the sacrifice.