Musnad Abu Ya'la al-Mawsili 209:39

Ubayd Allah ibn Mu'adh ibn Mu'adh narrated to us, my father narrated to us, Muhammad ibn 'Amr al-Laythi narrated to us, from 'Amr ibn Muslim ibn 'Ammar ibn Akimah, who said: I heard Sa'id ibn al-Musayyib saying: I heard Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever has a sacrificial animal to slaughter, then when the crescent of Dhu al-Hijjah appears, let him not take from his hair nor from his nails until he offers the sacrifice."