Roig's Guideline 5: Whether we are paraphrasing or summarizing we must always identify the source of the information.
No matter how different the words are that you're using, when you are paraphrasing other people's works, it's always a good idea to provide the source of your information. Miguel Roig, from St. John's University, lays out 24 guidelines for preventing plagiarism. Number 5 on the list stresses the importance of citing paraphrased materials.
"Guideline 5: Whether we are paraphrasing or summarizing we must always identify the source of the information. At other times, and for a variety of reasons, we may wish to restate in detail and in our own words a certain portion of another author’s writing. In this case, we must rely on the process of paraphrasing. Unlike a summary, which results in a substantially shorter textual product, a paraphrase usually results in writing of equivalent textual length as the original, but, of course, with a different words and, ideally, different sentence structure. Whether paraphrasing or summarizing others’ work, we must always provide proper credit. In fact, when paraphrasing in the humanities, one may thoroughly modify another author’s text and provide the proper citation. However, if the original sentence structure is preserved in the paraphrase, some will classify such writing as an instance of plagiarism.