Mage2\User\Requests\Admin\AdminUserRequest::rules PHP Method

    public function rules()
    {
        $validation['first_name'] = 'required|max:255';
        $validation['last_name'] = 'required|max:255';
        if ($this->getMethod() == 'POST') {
            $validation['email'] = 'required|email|max:255|unique:users';
            $validation['password'] = 'required|min:6|confirmed';
        }
        return $validation;
    }