The voice proceeded from a cloud in which God is wont to appear, that they might believe that the voice was sent forth from God. But in that He says, “This is My beloved Son,” He declares that the will of the Father and the Son is one, and that, save in that He is the Son, He is in all things One with Him who begot Him.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. in Matt., 56