5 U.S. Code § 3331 - Oath of office

https://www.law.cornell.edu/uscode/text/5/3331

5 U.S. Code § 3332 - Officer affidavit; no consideration paid for appointment

https://www.law.cornell.edu/uscode/text/5/3332

Here is the form they must sign.

https://www.opm.gov/forms/pdf_fill/sf61.pdf

5 U.S. Code § 3331 - Oath of office

https://www.law.cornell.edu/uscode/text/5/3331

5 U.S. Code § 3332 - Officer affidavit; no consideration paid for appointment

https://www.law.cornell.edu/uscode/text/5/3332