A credentialed reporter directly quoted by the UPI at the time is enough to give this quote credence until the claim can be disproven.
Otherwise, I have no problem with the senior Bush and have no reason to WANT anything bad about him. I know he's basically a good man. I fought under his command in a just war, and although I wish he'd finished the job I know his valid political and practical reasons for not doing so at the time.
Do you realize you are asking someone to prove he DIDN'T say something? Asking someone to disprove a negative is not how it works. You made the assertion, you need to prove it happened.
Do you wonder at all why, given the controversial and sensational nature of the quote, it can only be sourced in obscure places?
We all make mistakes and I think you are making one here, but it's one at George H. W. Bush's expense.