Sinan bin Salamah al-Hudhali narrates from his father, who had kept the company of the Prophet (peace and blessings be upon him), that the Prophet (peace and blessings be upon him) sent two sacrificial camels with a man and said: "Mark them at the place where they are slaughtered (above the chest), then dip the sandal in their blood and strike their sides with it, so that it is known that they are sacrificial camels."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6316