I've got a similar passage in Plato's Phaedrus if that helps:

But when it came to writing Theuth said, 'Here, O king, is a branch of learning that will make the people of Egypt wiser and improve their memories; my discovery provides a recipe for memory and wisdom.' But the king answered and said, 'O man full of arts, to one it is given to create the things of art, and to another to judge what measure of harm and of profit they have for those that shall employ them. And so it is that you, by reasons of your tender regard for the writing that is your offspring, have declared the very opposite of its true effect. If men learn this, it will implant forgetfulness in their souls; they will cease to exercise memory because they rely on that which is written, calling things to remembrance no longer from within themselves, but by means of external marks. What you have discovered is a recipe not for memory, but for reminder. And it is no true wisdom that you offer your disciples, but only its semblance, for by telling them of many things without teaching them you will make them seem to know much, while for the most part they know nothing, and as men filled, not with wisdom but with the conceit of wisdom, they will be a burden to their fellows.

He reiterates this stance in the Seventh Letter, in which he more or less returns to the ideas his Allegory of the Cave illustrates - basically, that writing adds a layer of obfuscation to our ideas and separates them further from truth, and that those who rely on the written word seek to collect knowledge instead of understanding it.

That said: Plato, of course, wrote extensively, and we would not have access to the vastness of his contributions to philosophy and metaphysics not the complexity of his thought without the words in which he set them down, so...

Are you maybe thinking of the story of the Egyptian gods Thoth/Theuth and Thamus, from Plato's Phaedrus? Theuth invents writing and Thamus says that those who use it "will be hearers of many things and will have learned nothing; they will appear to be omniscient and will generally know nothing; they will be tiresome company, having the show of wisdom without the reality."