be in black and white

To be in writing. Used especially in reference to an agreement, contract, or other formal document. It's right there in black and white that employees caught taking office supplies will be fired immediately. Please send me an email summarizing this conversation so it is all in black and white, should the issue come up again in the future. They promised to give me a raise when the project is complete, but I won't believe it until it's in black and white.