Many rulings and even laws in today's societies have a basis/root in the past.
For example, let's say that there is a community which holds a king from the 18th century, let's call them A, in a very high regard. Say, in 1850's, during the rule of A, a person who had a second marriage while their first spouse was alive, was prosecuted and punished. This particular incident in history serves as an evidence that A was against polygamy. The community might still have laws against polygamy, simply because the practice of polygamy has never been acceptable in the society, and this has more to do with A having been against polygamy, than the issue of human rights.
Now, I want to ask a question about a similar thing being practiced in a community.
I want to ask if there are any incidents in the history in which this particular thing was practiced (which became the root of the practice which still continues today)?
Is there a word which describes an incident from history which becomes a basis for some law, rule, custom being practiced today?