Fabian of Rome

For there was no laying of accusations against each other among them, except what was friendly; neither ought there ever to be such among their followers or among believers: for the Lord says, "Do not that to another which thou wouldst not have done to thyself." And He says also, "Thou shalt love thy neighbour as thyself;" and, "Love worketh no ill to his neighbour."

Decretals, Epistles of Pope Fabian, To All the Bishops of the East, Of the right of bishops not to be accused or hurt by detraction ↗

Augustine of Hippo

This virtue consists in nothing else but in loving what is worthy of love; it is prudence to choose this, fortitude to be turned from it by no obstacles, temperance to be enticed by no allurements, justice to be diverted by no pride. Why do we choose what we exclusively love, except that we find nothing better? But this is God, and if we prefer or equate any creature with God, we know nothing about loving ourselves. We are made better by approaching closer to him than whom nothing is better. …

Letter 155, To Macedonius ↗

Glossa Ordinaria

It goes on: And the second is like, namely this, Thou shalt love thy neighbour as thyself.

Catena Aurea by Aquinas ↗