Source of Golden Rule quote

Wikipedia says there's an Udanavarga -- which doesn't exist in the Pali canon, but which it compares with the Dhammapada.

The "rule" you're looking for is I think found in the "Violence" chapter of the Dhammapada, which starts like this:

129. All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.

130. All tremble at violence; life is dear to all. Putting oneself in the place of another, one should not kill nor cause another to kill.

131. One who, while himself seeking happiness, oppresses with violence other beings who also desire happiness, will not attain happiness hereafter.

132. One who, while himself seeking happiness, does not oppress with violence other beings who also desire happiness, will find happiness hereafter.

Once I started reading the suttas, I began to question the Golden Rule, which misses the mark on some very critical issues. The Golden Rule can degenerate into co-dependent dysfunctional relationships, and that bothered me. But when I read the suttas, the clarity and detail I found there addressed all my questions. The Golden Rule is, in other words, concise but imprecise.

Similar wisdom exists in the suttas, but with much finer detail. For example, notice that in the following sutta, the Buddha emphasizes benefit as well as reciprocity:

AN4.95:1.1: “Mendicants, these four people are found in the world.
AN4.95:1.2: What four?
AN4.95:1.3: One who practices to benefit neither themselves nor others;
AN4.95:1.4: one who practices to benefit others, but not themselves;
AN4.95:1.5: one who practices to benefit themselves, but not others; and
AN4.95:1.6: one who practices to benefit both themselves and others.
AN4.95:2.1: Suppose there was a firebrand for lighting a funeral pyre, burning at both ends, and smeared with dung in the middle. It couldn’t be used as timber either in the village or the wilderness.
AN4.95:2.2: The person who practices to benefit neither themselves nor others is like this, I say.
AN4.95:3.1: The person who practices to benefit others, but not themselves, is better than that.
AN4.95:3.2: The person who practices to benefit themselves, but not others, is better than both of those.
AN4.95:3.3: But the person who practices to benefit both themselves and others is the foremost, best, chief, highest, and finest of the four.

Notice that the word "benefit" is itself not defined here. And that is because to understand "benefit", we need to understand "right view".

In contrast, the Golden Rule is a bit too loose here with its paraphrase "as you would be treated." It's a bit too loose because the Golden Rule can be corrupted with simple thoughts such as "I am an addict who would like to be treated with drugs."

From a Buddhist perspective, wanting to be treated with drugs is wrong view. So I would look to the suttas for the wisdom that inspired the Golden Rule, and I would study that wisdom very carefully!

MN8:12.2: ‘Others will be cruel, but here we will not be cruel.’
MN8:12.3: ‘Others will kill living creatures, but here we will not kill living creatures.’
MN8:12.4: ‘Others will steal, but here we will not steal.’
MN8:12.5: ‘Others will be unchaste, but here we will not be unchaste.’
MN8:12.6: ‘Others will lie, but here we will not lie.’
MN8:12.7: ‘Others will speak divisively, but here we will not speak divisively.’
MN8:12.8: ‘Others will speak harshly, but here we will not speak harshly.’
MN8:12.9: ‘Others will talk nonsense, but here we will not talk nonsense.’
MN8:12.10: ‘Others will be covetous, but here we will not be covetous.’
MN8:12.11: ‘Others will have ill will, but here we will not have ill will.’
MN8:12.12: ‘Others will have wrong view, but here we will have right view.’
...

The Golden Rule is a great start. With the Golden Rule we consider others. And that is huge. For parents talking to very young children, the Golden Rule can present a valuable perspective for newly developing personalities. However, the Golden Rule also has dangerous loopholes that need to be avoided.

Or maybe the King

Searching all directions with your awareness, you find no one dearer than yourself. In the same way, others are thickly dear to themselves. So you shouldn't hurt others if you love yourself.

Lot of instance to be found, good householder.

As for "Hurt not others in ways that you yourself would find hurtful.": that's actually a western proverb made into a fake quote of the Buddha. Not all harm perceived by others, one would see as harmful in this way. So there are simply precepts, 'don'ts' to avoid dangers of such justifications. Would good householder feel well if an ma*****stic person uses such idea when on to hurt? Whould good householder feel lighter if someone hurting says he doesn't think he would be harmed by same?

Simple intended to not hurt, and stay attentive when acting, is surely more protective. (before doing "will it hurt myself or others", while, and later, reflective) Simple stop if hurtful and abound doing, if it was, do on and on if pleasing.