Dream on
Definition 1 of 2
Pro Tip 1/3Sarcasm Indicator
Often used sarcastically to indicate doubt or disbelief in someone's plan or goal. 
You think you're going to win the lottery? Dream on, pal.
Pro Tip 2/3Informal Tone
Mainly used in casual conversations, not in formal settings. 
Become a movie star overnight? Dream on, dude.
Pro Tip 3/3Mild Taunt
Can be a friendly taunt among friends or competitors. 
You think you'll beat me at chess? Dream on!