8 L. Thirsty men in their dreams eagerly gulp down the water of the stream, and the more they drink the thirstier they are. In the same way you appear to me to have searched everywhere for arguments against the point I raised, and yet to be as far as ever from being satisfied. Don't you know that the laying on of hands after baptism and then the invocation of the Holy Spirit is a custom of the Churches? Do you demand Scripture proof? You may find it in the Acts of the Apostles.
And even if it did not rest on the authority of Scripture the consensus of the whole world in this respect would have the force of a command. For many other observances of the Churches, which are due to tradition, have acquired the authority of the written law, as for instance the practice of dipping the head three times in the laver, and then, after leaving the water, of tasting mingled milk and honey in representation of infancy; and, again, the practices of standing up in worship on the Lord's day, and ceasing from fasting every Pentecost; and there are many other unwritten practices which have won their place through reason and custom. So you see we follow the practice of the Church, although it may be clear that a person was baptized before the Spirit was invoked.
Source: The Dialogue Against the Luciferians (New Advent)