Common knowledge does not need to be cited. Common knowledge includes facts that are known by a lot of people and can be found in many sources. For example, you do not need to cite the following:
Your own work. If it's your words, your opinion, your photo, or your graph, of course, you don't need to cite it.
Photo from https://commons.wikimedia.org/wiki/File:Abraham_Lincoln_November_1863.jpg