How to cite scientific laws
Common knowledge doesn't need to be cited. The threshold for what counts as "common" varies: My rule of thumb for a scientific publication is that if it is taught in the intro lectures of your discipline, it's common knowledge. I sometimes mention Planck's law at the start of a Methods section, and don't give a citation. Exceptions are of course if your paper is about the history of the law itself, or a related topic, in which case you would be interested in specific historical formulations and usage contexts anyway and should cite a source. (Or for example in the pedagogy of physics, if you want to compare textbook formulations - cite them.) Another case is if you use something that's common knowledge in a different discipline. I find this to be the case with statistical and machine learning techniques which a reader in my (geosciences) area might not be familiar with, but which would need no citation in a statistics paper. In this case, I found a widely used textbook to cite (with "e.g." before the reference).

References have a purpose. You can ask yourself: Who is my reader, and would they either know this or recognize it as background knowledge that they can look up in standard basic references? Pythagoras theorem - taught to every teen in the country, so needs no cite. 

For a class paper, you should probably ask the instructor about where the limit for cite-worthy lies. I've found that "what was taught in THIS class and its immediate predecessors" is usually considered common knowledge. If you want to cite something, a standard textbook will do.