God the Father said:
KJV Hebrews 1:8-12 But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom. Thou hast loved righteousness, and hated iniquity; therefore God, even thy God, hath anointed thee with the oil of gladness above thy fellows. And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands: They shall perish; but thou remainest; and they all shall wax old as doth a garment; And as a vesture shalt thou fold them up, and they shall be changed: but thou art the same, and thy years shall not fail.
Father testified to the preexistence of Christ. He has declared that Jesus is the Creator of heavens and earth.
Christ talking about his incarnation says in Psalms through David:
KJV Psalms 40:7-8 Then said I, Lo, I come: in the volume of the book it is written of me, I delight to do thy will, O my God: yea, thy law is within my heart.
Jesus said, concerning his preexistence during the patriarchal age:
John 8:56-58 Your father Abraham rejoiced to see my day: and he saw it, and was glad. Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham? Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
'Before' in the verse above is definitely in the context of time and not place or preeminence, as the question of the Jews clarifies it in verse 57.
Jesus said again:
KJV John 5:37 And the Father himself, which hath sent me, hath borne witness of me. Ye have neither heard his voice at any time, nor seen his shape.
Jesus was sent. This presupposes the preexistence of the one being sent. One can't send something which does not exist.
John the beloved witnessed:
KJV John 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.
Jesus having been in His Father's bosom, having seen the Father is able to declare regarding Him.
Jesus said in Revelation:
KJV Revelation 22:12-13 And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be. I am Alpha and Omega, the beginning and the end, the first and the last.
Jesus is the beginning and the end - the very words that the Father used in the first chapter of Revelation to qualify Himself.
Thus the preexistence of Christ is a theme that is woven across the Bible - both the Old and the New.