Musnad Ahmad 1156:4

Sufyan ibn 'Uyaynah narrated to us, from 'Abd al-Rahman ibn Humayd, who heard Sa'id ibn al-Musayyib, from Umm Salamah, from the Prophet, peace and blessings of Allah be upon him: "When the ten (days of Dhu al-Hijjah) enter, and a man intends to offer a sacrifice, let him not take from his hair nor from his skin."