Sahih Muslim Book 22, Hadith Number 4875.

Chapter : It is not permissible for one who intends to sacrifice the animal to get one’s hair or nails cut after the beginning of Dhull-Hijja.

Amr b. Muslim al-Jundani reported that Ibn Musayyib had told him that it was Umm Salama, the wife of Allah’s Apostle (may peace be upon him), who had informed him of that as narrated above.