When you entered the backup password if you have a Mac and have Keychain enabled you were given the option of saving the password in Keychain (this is the default). So check Keychain for it.
If you haven’t erased the phone yet you can create a new backup with a new password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support
If neither of those work you get unlimited guesses, so try to guess what password you might have entered back the first time you entered it (the password never changes from that first time, unless you explicitly request it be changed). If you are like most people it will be a password that you reused. And to speed up the process click to change the password and you will be prompted immediately.