dream on!

Dream on! {humorous} — мечтать не вредно; мечтать не запретишь; размечтался; ага, щас

Something that you say to someone who has just told you about something they are hoping for, in order to show that you do not believe it will happen.

Example: You think I'm going to help you move house? Dream on! — Думаешь я собираюсь помочь тебе переехать? Размечтался.(askhat4ever)

See also
[так я и поверил]
[fat chance]
[in your dreams]
[don't hold your breath]