believe something when you see it idiom
亲眼看见才会相信
If you say you will believe something when you see it, you mean you think it will not happen.
"Hetty says she'll be here on time." "I'll believe that when I see it." “赫蒂说她7点钟会来这儿。”“除非我亲眼见到，否则我是不会相信的。”