That Christ is called a sheep and a lamb who was to be slain, and concerning the sacrament (mystery) of the passion
In Isaiah: “He was led as a sheep to the slaughter, and as a lamb before his shearer is dumb, so He opened not His mouth. In His humiliation His judgment was taken away: who shall relate His nativity? Because His life shall be taken away from the earth. By the transgressions of my people He was led to death; and I will give the wicked for His burial, and the rich themselves for His death; because He did no wickedness, nor deceits with His mouth. Wherefore He shall gain many, and shall divide the spoils of the strong; because His soul was delivered up to death, and He was counted among transgressors.
And He bare the sins of many, and was delivered for their offenses.” Also in Jeremiah: “Lord, give me knowledge, and I shall know it: then I saw their meditations. I was led like a lamb without malice to the slaughter; against me they devised a device, saying, Come, let us cast the tree into His bread, and let us erase His life from the earth, and His name shall no more be a remembrance.” Also in Exodus God said to Moses: “Let them take to themselves each man a sheep, through the houses of the tribes, a sheep without blemish, perfect, male, of a year old it shall be to you.
You shall take it from the lambs and from the goats, and all the congregation of the synagogue of the children of Israel shall kill it in the evening; and they shall take of its blood, and shall place it upon the two posts, and upon the threshold in the houses, in the very houses in which they shall eat it. And they shall eat the flesh on the same night, roasted with fire; and they shall eat unleavened bread with bitter herbs. You shall not eat of them raw nor dressed in water, but roasted with fire; the head with the feet and the inward parts.
You shall leave nothing of them to the morning; and you shall not break a bone of it. But what of it shall be left to the morning shall be burnt with fire. But thus you shall eat it; your loins girt, and your sandals on your feet, and your staff in your hands; and you shall eat it in haste: for it is the Lord's passover.” Also in the Apocalypse: “And I saw in the midst of the throne, and of the four living creatures, and in the midst of the elders, a Lamb standing as if slain, having seven horns and seven eyes, which are the seven spirits of God sent forth throughout all the earth.
And He came and took the book from the right. hand of God, who sate on the throne. And when He had taken the book, the four living creatures and the four and twenty elders cast themselves before the Lamb, having every one of them harps and golden cups full of odours of supplications, which are the prayers of the saints; and they sang a new song, saying, Worthy are You, O Lord, to take the book, and to open its seals: for You were slain, and hast redeemed us with Your blood from every tribe, and people, and nation; and You have made us a kingdom unto our God, and hast made us priests, and they shall reign upon the earth.” Also in the Gospel: “On the next day John saw Jesus coming to him, and says, Behold the Lamb of God, and behold Him that takes away the sins of the world!”
Source: The Treatises of Cyprian (New Advent)