Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 313
Prohibition of having a Hair cut or paring one's nail during the first ten days of Dhul-Hijjah for one who intends to Sacrifice an Animal

1706. Umm Salamah (May Allah be pleased with her) said: The Messenger of Allah (PBUH) said, "When anyone of you intends to sacrifice the animal and enter in the month of Dhul-Hijjah, he should not get his hair cut or nails pared till he has offered his sacrifice.''
[Muslim].

Commentary: According to this Hadith, one who intends to sacrifice animal on `Eid Al-Adha, should abstain from paring nails, having a hair cut, and shaving the armpits and the private parts so that his sacrifice is in accordance with the Sunnah. One should have a hair cut on the tenth of Dhul-Hijjah after having offered the sacrifice. Some Ahadith tell us that if a person who does not sacrifice an animal on `Eid Al-Adha but pares his nails and has his hair cut on the tenth Dhul-Hijjah will be given by Allah the reward of sacrifice.