Ritual casting does not expend a spell slot, I know that. However, the Mage class feature does not say anything about whether you should prepare the subject spell to cast it as a ritual. On one side, it is anyway "casting" a spell, so you can say "yes, you should prepare." On the other hand, (although it is just about how mages cast rituals,) your ritual involves your spellbook, so you can see the ritual ceremony "from the spellbook", which removes the need to prepare the spell.
My question is: Should I prepare the subject spell in advance to perform it?
Edit: This is only about Mages. Clerics, Druids, Bards have these problems nailed down explicitly. Clerics' and Druids' ritual casting requires you to prepare the spell in advance, and Bards' rituals can only be drawn from what they know.