mongodb/docs — pull request #1788 by drmirror (2014-05-19 13:58:22+00:00)
Title: No need to created hashed index before sharding

It is not necessary to create a hashed index before defining a hashed shard key on it; the index is created automatically, just as with any other kind of shard key. The example proved confusing for a customer who thought that you always had to create the index first.