Purphoros’s Votary

Piety Trait

Piety 10+ Purphoros trait

You can cast heat metal with this trait, requiring no material components. Once you cast the spell in this way, you can’t do so again until you finish a long rest. Intelligence is your spellcasting ability for this spell.

In addition, you have advantage on saving throws against being knocked prone.

Source: Mythic Odysseys of Theros p. 76