In your dreams
Definition 1 of 1
Pro Tip 1/3Sarcastic Encouragement
Often used to sarcastically encourage someone to keep dreaming since it's unlikely to happen. 
A trip to Mars? In your dreams, pal!
Pro Tip 2/3Playful Mockery
Used among friends to playfully mock or tease them about unrealistic expectations. 
Me, doing your homework for you? In your dreams!
Pro Tip 3/3Dismissive Retort
Can be a dismissive retort, signaling that the idea presented is outlandish or fanciful. 
You managing the entire project alone? In your dreams!