Sayyiduna Anas bin Malik (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) sacrificed two rams that were greyish-white in colour and horned. He (peace and blessings be upon him) placed his foot on their necks and, while slaughtering them, recited 'Bismillahi wallahu akbar' (In the name of Allah, and Allah is the Greatest).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19008