Is there a source for the story about the sun being covered by the cloud?

A common analogy about mindfulness I hear about is that the wisdom is like the sun. It is always shining there, but is frequently clouded. Your job is to avoid as much cloud as possible, and when there is no cloud left, you are enlightened. Where is this analogy first use? Is there a sutta for this? If not, and if you have heard about this too, can you name the source that you first knew it?