Tertullian, On Baptism 8

Then the hand is laid on, calling and inviting the Holy Spirit through a blessing. Human ingenuity may claim the right to summon a spirit into the water and animate the union of the baptized by placing hands on them, using some other spirit of great clarity. So will God not be allowed to modulate spiritual sublimity through holy hands in His own instrument? This also comes from the old sacrament, by which Jacob blessed his grandchildren Ephraim and Manasseh, the sons of Joseph, by laying hands on their heads and crossing them over—turning them obliquely toward himself so that, in distorting the figure of Christ, they already foreshadowed the future blessing in Christ (Genesis 48:14). Then that most holy Spirit descends willingly from the Father upon the purified and blessed bodies, over the waters of Baptism, as if recognizing His former seat, and rests there, having come down in the form of a dove upon the Lord, so that the nature of the Holy Spirit might be declared through an animal of simplicity and innocence—a dove that even physically lacks bile. And therefore: 'Be simple as doves,' He says; not even this command is without support from the preceding figure. For just as after the waters of the flood, by which ancient iniquity was purged, after Baptism, so to speak, the dove—a herald of heavenly wrath—announced peace to the earth when sent out from the ark and returned with an olive branch, a sign still held out among nations as a pledge of peace: by the same arrangement, the dove of the Holy Spirit flies to the spiritual effect on the earth, that is, on our flesh emerging from the washing after former sins, bringing the peace of God, sent from heaven where the Church is figured as the ark. But the world sins again, which makes Baptism compare poorly with the flood. And so it is destined for fire, just as the man who restores his sins after Baptism is; this too must be taken as a sign of our warning.