When should I cite, if using code from Stack Overflow?

I already found a question handling the case if you got some information from Stack Exchange for my thesis. Unfortunately I did not find anything about the case that I get a solution (not necessarily from stackexchange) for a specific problem.

For example, if I have a problem with a specific part in my code, or if I have to design a circuit, and I run into a problem: What is the threshold of information, i.e. when should I cite the answer in my thesis? Should I always cite Stack Overflow (or the other web page), even if its just for two lines? Where is the threshold?

And how should I do this in my code? Should I rather make a comment block around the specific part of the code, redirecting to SO, or should I simply cite it in my thesis?