It was He who was seen by the king of Babylon in the furnace with His martyrs: “the fourth, who was like the Son of man.” He also was revealed to Daniel himself expressly as “the Son of man, coming in the clouds of heaven” as a Judge, as also the Scripture shows. What I have advanced might have been sufficient concerning the designation in prophecy of the Son of man. But the Scripture offers me further information, even in the interpretation of the Lord Himself. For when the Jews, who looked at Him as merely man, and were not yet sure that He was God also, as being likewise the Son of God, rightly enough said that a man could not forgive sins, but God alone, why did He not, following up their point about man, answer them, that He had power to remit sins; inasmuch as, when He mentioned the Son of man, He also named a human being? except it were because He wanted, by help of the very designation “Son of man” from the book of Daniel, so to induce them to reflect as to show them that He who remitted sins was God and man…
Tertullian
The Five Books Against Marcion, Book IV
Chapter X