I'll believe it/that when I see it
I'll believe it when I see it
I highly doubt that could happen or is the case. The phrase implies that one would need to witness or see proof of such a thing to believe it. The kids said they're finished cleaning the garage, but I'll believe it when I see it. A: "He's actually changed a lot since college. He's very nice now." B: "I'll believe it when I see it."