From On the Trinity:
But since He so took the form of a servant, as that the unchangeable form of God remained, it is clear that that which became apparent in the Son was done by the Father and the Son not being apparent; that is, that by the invisible Father, with the invisible Son, the same Son Himself was sent so as to be visible. Why, therefore, does He say, Neither came I of myself? This, we may now say, is said according to the form of a servant, in the same way as it is said, I judge no man.
10. If, therefore, He is said to be sent, in so far as He appeared outwardly in the bodily creature, who inwardly in His spiritual nature is always hidden from the eyes of mortals, it is now easy to understand also of the Holy Spirit why He too is said to be sent. For in due time a certain outward appearance of the creature was wrought, wherein the Holy Spirit might be visibly shown; whether when He descended upon the Lord Himself in a bodily shape as a dove, or when, ten days having past since His ascension, on the day of Pentecost a sound came suddenly from heaven as of a rushing mighty wind, and cloven tongues like as of fire were seen upon them, and it sat upon each of them. This operation, visibly exhibited, and presented to mortal eyes, is called the sending of the Holy Spirit; not that His very substance appeared, in which He himself also is invisible and unchangeable, like the Father and the Son, but that the hearts of men, touched by things seen outwardly, might be turned from the manifestation in time of Him as coming to His hidden eternity as ever present.