The Constitutional Provision Respecting Copyright:
"The Congress shall have Power…To promote the Progress of Science and useful Arts, by securing for limited Tımes to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
-United States Constitution, Article I, Section 8
Credit must be given to any work you quote from, borrow from, or in any way include in your own work. If you did not create, write, or fashion a work entirely yourself, you must give credit. Even to anonymous creators in history!