No, in the same way that I wouldn't cite Euler every time I used one of his theorems - because it's so well known that its history is well documented in countless places.

However, if I was using a more recent/niche/unknown theorem, it would absolutely be considered bad practice not to cite where I got it from.

If I was implementing any known (named) algorithm intentionally I think I would absolutely say so in a comment (`// here we use quick sort to...` and maybe why it's the choice) and then it's easy for someone to look up and see it's due to Hoare or whoever on Wikipedia etc.

Now many will downvote you because this is an algorithm and not some code. But the reality is that programming is in large part built looking at somebody else code / techniques, internalizing them, and reproducing them again with changes. So actually it works like that for code as well.