The only source material that you can use in an essay without attribution is material that is considered common knowledge and is therefore not attributable to one source. Common knowledge is information generally known to an educated reader, such as widely known facts and dates, and, more rarely, ideas or language. Facts, ideas, and language that are distinct and unique products of a particular individual's work do not count as common knowledge and must always be cited. Figuring out whether something is common knowledge can be tricky, and it's always better to cite a source if you're not sure whether the information or idea is common knowledge. If you err on the side of caution, the worst outcome would be that an instructor would tell you that you didn't need to cite; if you don't cite, you could end up with a larger problem.