A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Usually cite a source for ideas not your own
Drafting your report
Part I. Research and writing: from planning to production

This rule is more complicated than it seems, because most of our own ideas are based on or derived from identifiable sources somewhere in history. Readers don't expect you to find every distant source for every familiar idea, but they do expect you to cite the source for an idea when (1) the idea is associated with a specific person and (2) it's new enough not to be part of a field's common knowledge.

For example, psychologists claim that we think and feel in different parts of our brains. But no reader would expect you to cite that idea, because it's no longer associated with a specific source and it's so familiar that no one would think you implied that it was yours. On the other hand, some psychologists argue that emotions are crucial to rational decision making. That idea is so new and so closely tied to particular researchers that you'd have to cite them.

The principle is this: cite a source for an idea not your own whenever an informed reader might think that you're implying that it is your own. Though that seems black and white, it has a big gray area in the middle. When in doubt, check with your instructor.