Testament of Benjamin (c. 1st century BC) calls the Messiah the "Lamb of God."
Testament of Benjamin 3 in H. C. Kee (trans.), "Testaments of the Twelve Patriarchs," The Old Testament Pseudepigrapha, ed. James H. Charlesworth (New York: Yale University Press, 1983), 1:825-26
3 1 “Now, my children, love the Lord God of heaven and earth; keep his commandments; pattern your life after the good and pious man Joseph. 2 Let your thoughts incline to the good, as you know to be so with me, because he who has the right set of mind sees everything rightly. 3 Fear the Lord and love your neighbor. Even if the spirits of Beliar seek to derange you with all sorts of wicked oppression, they will not dominate you, any more than they dominated Joseph, my brother. 4 How many men wanted to destroy him, and God looked out for him! For the person who fears God and loves his neighbor cannot be plagued by the spirit of Beliar since he is sheltered by the fear of God. 5 Neither man’s schemes not those of animals can prevail over him, for he is aided in living by this: by the love which he has toward his neighbor. 6 Joseph also urged our father to pray for his brothers, that the Lord would not hold them accountable for their sin which they so wickedly committed against him. 7 And Jacob cried out, ‘O noble child, you have crushed the inner feelings of Jacob, your father.’ He embraced him and kept kissing him for two hours, saying,
8 ‘Through you will be fulfilled the heavenly prophecy concerning the Lamb of God, the Savior of the world, because the unspotted one will be betrayed by lawless men, and the sinless one will die for impious men by the blood of the covenant for the salvation of the gentiles and of Israel and the destruction of Beliar and his servants.’