There isn't any single document stating the rights of somebody who has a Family Permit. You could try to persuade your future employer that you have the right to work by citing
Regulation 7 of the EEA Regulations 2016.
But a Family Permit is not a statutory excuse listed in
An Employer's Guide to Right of Work Checks, which would make them very nervous of hiring you.
Remember that the Family Permit is primarily an entry clearance document (for you to enter the country), not a residence document.
If you are the spouse or child of an EEA citizen currently resident in the UK, you would get a CoA letter stating your right to work in the UK when you apply for a Residence Card.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.