get (something) in writing

To have some detail, condition, agreement, etc., put into a printed, written, or typed document, typically in order to prove its existence at a later point in time. It sounds like they're offering a really good discount on the car. Just make sure you get it in writing so dealer can't go back on it later. I make sure to get every detail of my terms of employment in writing before I sign a contract. That way I have something concrete to point to should there be any dispute in the future. That's a pretty generous offer—can we get it in writing?