The First book of Enoch is one of the Old Testament pseudepigrapha, in other words, a composition attributed to a personage from the Hebrew Bible, but not included in either the Jewish or the Christian Bible. It contains a description of the Son of Man as pre-existent.
[1 Enoch.46.1-4] And there I saw the One to Whom belongs the time before time, and His head was white like wool. With Him was another being, whose countenance had the appearance of a man, and his face was full of graciousness, like one of the holy angels. I asked the angel who went with me [...] concerning that son of and who he was, and whence he was, and why he went with the One to Whom belongs the time before time.
He answered and said to me: 'This is the son of man who has righteousness, with whom dwells righteousness, and who reveals all the treasures of that which is hidden, because the Lord of the spirits has chosen him, and whose lot has the pre-eminence before the Lord of the spirits in uprightness for ever. This son of man whom you have seen shall raise up the kings and the mighty from their seats and the strong from their thrones, and shall loosen the reins of the strong and break the teeth of the sinners.'
[1 Enoch.48.2-10] And at that hour that Son of Man was named in the presence of the Lord of the spirits, and his name before the the One to Whom belongs the time before time. Yes, before the sun and the signs were created, before the stars of the heaven were made, his name was named before the Lord of the spirits. He shall be a staff to the righteous whereon to stay themselves and not fall, and he shall be the light of the gentiles and the hope of those who are troubled of heart. All who dwell on earth shall fall down and worship before him, and will praise and bless and celebrate with song the Lord of the spirits. For this reason has he been chosen and hidden before Him, before the creation of the world and for ever more. The wisdom of the Lord of the spirits has revealed him to the holy and righteous; for he has preserved the lot of the righteous, because they have hated and despised this world of unrighteousness, and have hated all its works and ways in the name of the Lord of the spirits: for in his name they are saved, and according to his good pleasure has it been in regard to their life.
In these days downcast in countenance shall the kings of the earth have become, and the strong who possess the land because of the works of their hands, for on the day of their anguish and affliction they shall not be able to save themselves. And I will give them over into the hands of My elect: as straw in the fire so shall they burn before the face of the holy, as lead in the water shall they sink before the face of the righteous, and no trace of them shall any more be found.
And on the day of their affliction there shall be rest on the earth, and before them they shall fall and not rise again. There shall be no one to take them with his hands and raise them, for they have denied the Lord of the spirits and His Messiah. The name of the Lord of the spirits be blessed.