Bùɡuān nǐde shì.
不关你的事。
It's none of your business.
Notes:
不(bù): not
关(ɡuān): be relative to
你的(nǐde): your
事(shì): matter,thing
Example:
A:你明天去参加晚会吗?
Nǐ mínɡtiān qù cānjiā wǎnhuì mɑ?
Will you attend the party tomorrow?
B:不关你的事。
Bùɡuān nǐde shì.
It's none of your business.