It is an unfair question since it requires anyone who responds to consider all the various sources of Church teaching, in chronological order, and find the earliest. It takes only a moment to ask the question, but it takes much research and time to answer it.
So, I’m just going to continue the discussion without stating the earliest example of that point.
The Council of Trent:
“By which words, a description of the Justification of the impious is indicated, – as being a translation, from that state wherein man is born a child of the first Adam, to the state of grace, and of the adoption of the sons of God, through the second Adam, Jesus Christ, our Saviour. And this translation, since the promulgation of the Gospel, cannot be effected, without the laver [washing] of regeneration, or the desire thereof, as it is written; unless a man be born again of water and the Holy Ghost, he cannot enter into the Kingdom of God.”
The laver of regernation is baptism; notice that the Council also taught a Baptism of desire, i.e. a non-formal Baptism.