The Sunnah indicates that the one who wants to offer a sacrifice must
refrain from taking anything from his hair, nails or skin from the first day
of Dhu’l-Hijjah until he offers his sacrifice, because the Prophet said:
“When you see the new moon of Dhu’l-Hijjah, if any one of you wants to offer
a sacrifice, let him not remove anything from his hair or nails until he has
offered his sacrifice.” (Reported by Muslim with four isnaads, 13/146).
This command implies obligation and the prohibition implies that it is
forbidden. If a person deliberately takes something (from his hair or
nails), he must seek the forgiveness of Allaah, but he does not have to pay
any fidyah (penalty), and his udhiyah is still valid. Whoever needs to
remove some of his hair or nails because leaving it will cause him harm,
such as a torn nail or a wound in a site covered by hair, should remove it,
and there is no sin on him if he does so. There is nothing wrong with men
and women washing their hair during the first ten days of Dhu’l-Hijjah.
The wisdom behind the prohibition on removing hair and nails is because the
one who is going to offer a sacrifice is like the one who is in ihraam for
Hajj and 'Umrah with regard to some rituals, which is the offering of a
sacrifice in order to draw closer to Allaah. Thus some of the rulings of
ihraam apply to the one who wants to offer a sacrifice, so he should not
remove his hair and nails until he has slaughtered his sacrifice, in the
hope that Allaah will release him from the fire of Hell. And Allaah knows
best.
If a person removes some of his hair and nails during the first ten days of
Dhu’l-Hijjah because he is not planning to offer a sacrifice, then he
decides to sacrifice, he should refrain from cutting his hair or nails from
the moment he takes the decision.
And Allah knows best.