Musnad Ahmad 795:11

Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, Abd al-Karim ibn Abi al-Mukhariq informed me, from Mu'adh ibn Sa'wah al-Rasibi, from Sinan ibn Salamah al-Hudhali, from his father Salamah—who had accompanied the Prophet (peace be upon him)—that the Prophet (peace be upon him) sent two sacrificial camels with a man and said: "If they become incapacitated, slaughter them, dip the sandal in their blood, then strike their sides with it so that it may be known that they are sacrificial camels." He said: "The sides of each one of them." He said: "Do not eat from them, neither you nor anyone from your companions, and leave them for those who come after you."