My DM does not believe that you can use a Warlock's spell slots to cast other casting classes' spells at your Warlock spell slot level, except via the feat Magic Initiate. How can I convince them that you can? Where is this explained in the core books?
For example: you are a Bard 1/Warlock 5. Can you cast the Bard Cure Wounds spell using one of your two 3rd-level Warlock spell slots? Does the Cure Wounds heal 3d8 + Cha mod instead of 1d8 + Cha mod? If so, where in the PHB or DMG can I find this?