Midrash sur Les Nombres 6:23

(Fol. 49) We are taught in a Baraitha concerning the passage (Num. 6, 23) On this wise ye shall bless the children of Israel. R. Ishmael says: "We learn from here that the blessing upon Israel comes through the priests, but we have not learned from where comes the blessings upon the priests themselves? Since the verse says (Ib.) And I will bless them, it should be understood that the priests will bless Israel and the Holy One, praised be He! will bless the priests." R. Akiba says: "We learn from the above passage that the blessing upon Israel comes only through the priests, but not from the Almighty. Since, however, the passage says, And I will bless them, it is understood that the priests bless Israel and the Holy One, praised be He! gives His consent to it." But whence does R. Akiba infer the blessings for the priests themselves? Said R. Nachman b. Isaac: "From (Gen. 12, 3) And I will bless them that bless thee." And thy sign [not to exchange R. Akiba's opinion for R. Ishmael's], is R. Ishmael, who is a priest himself, supports the priest [contending that the last part of the blessings refers to the priests instead of to Israel].

From whom did the priests receive the privilege of pronouncing the blessing over Israel? R. Eleazer the son of Azariah said: They received this privilege from Jacob, concerning whom it is written: And the lad will go yonder(koh) (Gen. 22:5). The word koh is also contained in the words spoken by Moses to the priests, as it is said: Thus (koh) shall ye bless (Num. 6:22). The rabbis maintained: They obtained this right at the time of the giving of the Torah, since it is said there: Thus (koh) shalt thou say to the house of Jacob (Exod. 19:3).