Is it a Sunnah not to cut the nails and beard for ten days if the person is sacrificing an animal on his behalf on 'Eed al- Adhha?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
Everyone who intends to offer a sacrifice has to refrain from clipping nails and cutting hair after beginning the first ten days of Thul-Hijjah. It is forbidden for such a person to cut his hair or nails. The majority of the Muslim scholars believe that it is merely detestable for the Hadeeth narrated by Umm Salamah, may Allah be pleased with her, that the Prophet said: "When the first ten days of Thul-Hijjah starts, one who intends to offer a sacrifice should not have his hair cut or his nails trimmed, till he/she has offered his/her sacrifice". [Muslim] It is also reported in a narration that he should not remove any of his skin.
The wisdom behind the above ruling is for all the parts of body to remain freed from the Hell.
However, if one cuts his/her hair or nails he/she has to ask forgiveness of Allah only and has to pay no expiation.
Allah knows best.
Fatwa answered by: The Fatwa Center at Islamweb