Μαθθαῖον 10·27 ὃ λέγω ὑμῖν ἐν τῇ σκοτίᾳ εἴπατε ἐν τῷ φωτί, καὶ ὃ εἰς τὸ οὖς ἀκούετε κηρύξατε ἐπὶ τῶν δωμάτων.
What I say to you in the dark, speak it in the light, and what you hear in your ear, proclaim it on the housetop. Matthew 10:27
Here are two* wonderful idioms that refer to something being told in secret. The first one may be close to our idiom "behind closed doors" and the second one is very close to our idiom “whisper in your ear”. The key to understanding this verse comes in the prior verse (26) So, if we were to put this into our own idiomatic way to speaking, we may say,
“What I say to you behind closed doors, proclaim it openly, and what you hear whispered in your ear, proclaim it out loud.”
*There are really four idioms in this verse.