none of (one's) business
Something that is of no concern to another. It's really none of his business what I do with the money I earn.
(It's) none of your business!
It is nothing that you need to know. It is none of your concern. (Not very polite.) Alice: How much does a little diamond like that cost? Mary: None of your business! John: Do you want to go out with me Friday night? Mary: Sorry, I don't think so. John: Well, what are you doing then? Mary: None of your business!