Rules for Morph

[Anonymous]

I was playing a game tonight and a friend did something I've never seen before.  He had a creature out face up and he paid 3 colorless to turn it face down.  He says he can do that so I went online to look up the rules for morphed creatures and I saw nothing that specified wether he could or couldnt do that.  I'm guess he cant but I'd appreciate it if someone could confirm wether I am right or not thanks.

[upinthe]

You cannot turn a face up creature with morph that is already in play face down unless a card instructs you to do so.