A factory reset, unless your bank means something completely different, is just a reinstallation of iOS on your iPhone. There will be no "receipt" of any kind.
I've never heard of any bank ever asking someone to restore their phone as a condition of unlocking a bank account, but in any case, you will need to get back with your bank and clarify what it is they are asking you to do. If they insist on some sort of written "proof" that you've reinstalled the operating system on your iPhone, then I don't know what you can do, since there is no "proof" available unless you can take the iPhone to a service center, have them do the reinstallation, and then get a letter from them stating that they've done so. Whether that would satisfy your bank I have no idea.
Regards.