Nānā i ke kumu
Citing the work that supports your research is both an ethical issue and a legal issue.
The ethics of citing is based on the scholarly tradition of giving credit for information and ideas that are not one's own. Science has a long tradition of acknowledging priority through citations. You have the ethical responsibility to cite all works that were used to support your research, to give credit to the earlier work and to provide a clear path for those who follow in your footsteps.
Or, to put it another way:
An example of some legal aspects of citing can be found in Copyrights, Title 17 of the U.S. Code, which describes the legal protections of authors' and creators' intellectual property rights within U.S. law.
*Pukui MK. 1983. ʻŌlelo Noʻeau : Hawaiian Proverbs & Poetical Sayings. Honolulu: Bishop Museum Press.  p.137.
Plagiarism is defined as:
"The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft"
"plagiarism, n." OED Online. Oxford University Press, March 2015. Web. 21 May 2015. Available from: http://www.oed.com/view/Entry/144939
See the University of Hawaiʻi Student Conduct Code for more information.