The general procedure is to look at a recent book or survey article that mentions this and see what is cited. If not the original author, then perhaps an earlier article. Then look in there, see what is cited, etc.
The hard part is knowing when to stop. If I cannot find the original author after a few hours of searching I would stop unless I am finding the process interesting and I have the time. It is acceptable to stop after a reasonable length of time and just cite a textbook or survey article.
You need to know your audience. I find many mathematicians love to read about a 150 year old paper in Flemish, while most physicists are not so historically minded. Nobody is right, expect Referee B who asks you to dig further back.