Steady Spellcasting Feat 6BardClericDruidOracleSorcererWitchWizardSource Advanced Player's Guide pg. 80,
Core Rulebook pg. 211 1.1
Confident in your technique, you don’t easily lose your concentration when you Cast a Spell. If a reaction would disrupt your spellcasting action, attempt a DC 15 flat check. If you succeed, your action isn’t disrupted.