I'll believe it 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."
See also: believe, see
Farlex Dictionary of Idioms.

I’ll believe it/that when I see it

(informal) used for expressing doubt that something will happen or be done: ‘He says he’s going to give up smoking.’ ‘I’ll believe that when I see it!’
See also: believe, see, that
Farlex Partner Idioms Dictionary
See also:
  • I'll believe it/that when I see it
  • blessed with (something)
  • throw (someone or something) on the scrap heap
  • send (one) on (one's) way
  • add up to the same thing
  • amount to the same thing
  • amount/come to the same thing
  • know too much
  • do as I say
  • do as I say, not as I do