Musnad Ahmad 25:626

Al-Hasan ibn Yahya narrated to us, Al-Fadl ibn Musa narrated to us, from Husayn ibn Waqid, from 'Ilba' ibn Ahmar, from 'Ikrimah, from Ibn 'Abbas, who said: "We were with the Prophet, may Allah bless him and grant him peace, on a journey, and the time of sacrifice (Nahr) arrived. So, we slaughtered a cow on behalf of seven (people), and a camel on behalf of ten (people)."