For example, a paladin of the ancients learns the ritual spell Speak with animals as an oath spell and thus has it always prepared. If I multiclass as Wizard, I gain the ability to cast ritual spells as rituals. Can I cast Speak with animals as a ritual?
Now, if I multiclass as Druid instead of Wizard, I know the spell as a druid spell but still have it prepared as a paladin spell. Can I cast it as a ritual now? And if yes, which spellcasting ability do I use for it?
Best Answer
No
As a wizard you gain the ability to cast wizard ritual spells as rituals, see ritual casting feature PHB page 114:
And the spellcasting subsection of multiclassing in the PHB says they are separate sets of spells for each class (page 164):
The druid ritual casting feature is worded in a similar manner, so that they can only ritual cast druid spells.