Feats

Boon of Spell Recall

Prerequisite
Level 19+, Spellcasting or Pact Magic feature
Ability Score Increase
+1 Intelligence or Wisdom or Charisma (max 30)
Source
XPHB

You gain the following benefits.

Free Casting

Whenever you cast a spell with a level 1-4 spell slot, roll 1d4. If the number you roll is the same as the slot's level, the slot isn't expended.