You think I will lend you my car? In your dreams.
in your dreams
idiomUsed to express the speaker's skepticism about another's preceding statement about a desired or assumed state of affairs.
He told me he would win the lottery tomorrow, but I told him he was living in his dreams.
When the rival candidate suggested he would easily secure the majority of the vote, the opposition leader simply laughed and replied that such a victory existed only in his dreams.