Musnad Ahmad 1156:182

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Malik ibn Anas, from ‘Umar or ‘Amr ibn Muslim, from Sa‘id ibn al-Musayyib, from Umm Salamah, from the Prophet, peace and blessings of Allah be upon him, that he said: "Whoever intends to sacrifice [an animal] at the sighting of the crescent of Dhu al-Hijjah, let him not take anything from his hair or his nails."