Tiānshàng bù huì diào xiànbǐng, nǎlǐ lái de héngcái?
There's no such thing as a free lunch — where does this windfall come from?
Tips
usage
横财 often implies money obtained through illegitimate or unexpected means. It carries a slightly negative connotation in most contexts, suggesting the money wasn't earned through honest work. Compare 意外之财 (unexpected fortune) which is more neutral.