Irenaeus [AH 3.15.3] "Knowing one and the same Son of God, Jesus Christ, who was announced by the prophets, who from the fruit of David’s body was Emmanuel, “the messenger of great counsel of the Father;” (Isa. ix. 6 LXX)
Tertullian [Against Marcion 3:10] "Similarly, again, Isaiah says: “For a child is born to us, and to us is given a son.” (Isa. ix. 6). What novelty is that, unless he is speaking of the “Son” of God?"