Roman Catechism 5.13

Chapter 5 — Part I: The Creed — Article III: Who Was Conceived by the Holy Ghost, Born of the Virgin Mary

Section 13. Although possessing that Spirit by which holy men attain the adoption of sons of God, He cannot, however, be called the adopted son of God; for since He is the Son of God by nature, the grace, or name of adoption, can on no account be deemed applicable to Him.