Remove image classes from post thumbnail output

I wish to remove the image classes generated by default in the output whenever post_thumbnail() is called –

<img width="1024" height="768" src="http://example.com/wp-content/uploads/2013/01/example-1024x768.jpg" class="attachment-large wp-post-image" alt="Yet another example">

I’ve read a bit about the remove_action() filter and this seems the way to go but I’m not sure how to use it.

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