According to the hashed index documentaion You can't!
MongoDB supports hashed indexes of any single field. The hashing function collapses sub-documents and computes the hash for the entire value, but does not support multi-key (i.e. arrays) indexes.
You may not create compound indexes that have hashed index fields
PS: The above is valid for versions 2.4 and 2.6 (which is the latest at the moment)
PS2: According to @naman's answer it is now possible in version 4.4