Musnad Abu Ya'la al-Mawsili 209:32

Nasr ibn Ali narrated to us, Muhammad ibn Abi Adi narrated to us, Muhammad ibn Amr narrated to us, from Amr ibn Muslim, from Sa’id ibn al-Musayyib, from Umm Salamah, from the Prophet (peace be upon him), who said: "Whoever sees the crescent of Dhu al-Hijjah and intends to offer a sacrifice, let him refrain from [cutting] his hair and his nails."