Some producers using public domain and fair use materials wonder what attribution they are required to give to the creators of those works, whether they must identify the works as being public domain or fair use, and what other rules they must follow when using such works.
If the material is in the public domain (meaning the copyright in the work has expired or never existed), copyright law places no obligation on you to refer to the work as public domain, to cite the source, or to provide any other type of credit. Likewise, if your use of the material qualifies as a fair use (here I’m referring to fair use as defined under Section 107 of the Copyright Act), copyright law places no obligation on you to indicate that you’re relying on fair use, to cite the source, or to provide any other type of credit.
Nevertheless, it is “courteous” to cite the source and/or provide a credit. Also, depending on your professional field, your colleagues may take a dim view of your quoting sources and using materials without providing an appropriate credit.
There is at least one caveat to providing credit when you are relying on fair use. Your claim that your use is protected by copyright fair use is not a guarantee that the rights owner will agree with you. There is no bright line in determining what qualifies as a fair use and what doesn’t. That's why the decision of whether to use a particular copyrighted work without permission often revolves around a risk assessment. One of the risk factors to consider is the likelihood of the use coming to the attention of the rights owner. Putting in a credit increases the likelihood that the rights owner will become aware of your use.
Because fair use is uncertain, it’s typically my preference to have permission rather than rely on fair use. If you have permission from the rights owner, you have a license from the rights owner to use the work in a particular manner. If the rights owner dictates a specific form of credit, I would characterize that specification as a term of the license agreement and I would do my best to comply.