Uninformed and incompetent people leading others who are similarly incapable.

This appears in the Bible, Matthew 15:14 (King James Version):

Let them alone: they be blind leaders of the blind. And if the blind lead the blind, both shall fall into the ditch.

Biblical citations usually tend to be the earliest. In this case the thought was probably inherited from the Upanishads - the sacred Hindu treatises, written 800-200 BC. From Katha Upanishad we have:

Abiding in the midst of ignorance, thinking themselves wise and learned, fools go aimlessly hither and thither, like blind led by the blind.