I said that somewhat tongue-in-cheek. In that part of the story yhwh instructed Moses to make another set of stone tablets for the 10 commandments. Moses was known as "Butterfingers" to his family, and he dropped the original set, which I believe yhwh also told him to write down.
So, in a sense, yhwh did say to write stuff down, specifically, yhwh's laws. It did not say to write "The Bible", since that was not even a concept at the alleged time of Moses. So you are right that you will not find that imperative in the bible. But you will find instances of yhwh demanding that laws be written down.