Unable To Restore Downgrade

Hi all I'm having issues restoring my iPhone and starting with a fresh software.

Im using my Mac which is up to date, I have managed to get the phone into recovery and hit restore and update and it will download the software to about 75% and then this message pops up, I have tried this many times with the same outcome, even with another Mac.

I know you can download the software online and with some key combinations click on restore and select the file but where is a trusted source to get the iOS 16.5.1 update from?