Backfire

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 5th level or lower, its spell fails and has no effect. If it is casting a spell of 6th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Additionally, if the spell is successfully interrupted, the creature must succeed on a Wisdom, Charisma, or Intelligence saving throw (thier choice) or become unable to speak or hear, immune to thunder damage, and incapable of casting a spell that requires a verbal component until the end of their next turn, as the magical energy is redirected to silence the caster of the interrupted spell.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.