On your side, however, you must make Him out to be a liar, and an impostor, and a tamperer with His word, if, when He was Himself a Son to Himself, He assigned the part of His Son to be played by another, when all the Scriptures attest the clear existence of, and distinction in (the Persons of) the Trinity, and indeed furnish us with our Rule of faith, that He who speaks, and He of whom He speaks, and to whom He speaks, cannot possibly seem to be One and the Same.
Still, in these few quotations the distinction of Persons in the Trinity is clearly set forth. For there is the Spirit Himself who speaks, and the Father to whom He speaks, and the Son of whom He speaks. In the same manner, the other passages also establish each one of several Persons in His special character—addressed as they in some cases are to the Father or to the Son respecting the Son, in other cases to the Son or to the Father concerning the Father, and again in other instances to the (Holy) Spirit.
Tertullian
Against Praxeas
Chapter XI