… When one cultivates to the utmost the principles of his nature, and exercises them on the principle of reciprocity, he is not far from the path. What you do not like when done to yourself, do not do to others. …
忠恕違道不遠，施諸己而不愿，亦勿施於人。
Doctrine of the Mean 13
The same idea in other traditions
And as ye would that men should do to you, do ye also to them likewise.
Let none deceive another, nor look down on anyone anywhere. Though provoked or aggrieved, let them not wish pain on each other.
Indifferent to worldly objects, a man should wander about treating all creatures in the world so as he himself would be treated.
Thou shalt not take vengeance, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
Do not utter even a single harsh word; your True Lord and Master abides in all. Do not break anyone's heart; these are all priceless jewels. The minds of all are like precious jewels; to harm them is not good at all. If you desire your Beloved, then do not break anyone's heart.