Musnad Ahmad 202:1

Waki' narrated to us, he said: Shu'bah narrated to us, from 'Amr ibn Murrah, from Murrah al-Tayyib, he said: A man from the companions of the Prophet, may Allah bless him and grant him peace, narrated to me in this room of mine—I think he said: The Messenger of Allah, may Allah bless him and grant him peace, delivered a sermon to us on the Day of Sacrifice while upon a red, slit-eared she-camel of his. He said: "This is the Day of Sacrifice, and this is the Day of the Greatest Hajj."