Musnad Ahmad 28:467

Yazid narrated to us, Al-Hajjaj informed us, from ‘Ata’, and from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, with the like of it from the Prophet, may Allah bless him and grant him peace, and he added: "A camel." And ‘Amr said in his hadith: "And he commanded him to fast a day in its place."