Sahih Muslim
The Book of Sacrifices (Kitab Al-Adahi)
Book Number: 022 / Hadith Number: 4841
Anas reported that Allah's Messenger (may peace be upon him) sacrificed with his own hands two horned rams which were white with black markings reciting the name of Allah and glorifying Him (saying Allah-o-Akbar). He placed his foot on their sides (while sacrificing).