Cutting the hair and nails for the one who wants to sacrifice

One is to not cut the nails or hair if one intends to slaughter. Imaam Ahmad considers it haraam for a person cut their nails or hair. Imaam al-Shaafi'i considers it disliked. 

Abu Hanifah has been reported to have had held the incorrect opinion that one can cut the hair and nails. In doing so he opposed the explicit authentic hadith mentioned below:

Umm Salama, the wife of Allah's Apostle (sallallaahu alayhi wa sallam), reported Allah's Messenger (sallallaahu alayhi wa sallam) 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 Hadith No. 1977]