Roman Catechism 17.45

Chapter 17 — Part II: The Sacraments — Confirmation

Section 45. The Slap On The Cheek — The person when anointed and confirmed next receives a gentle slap on the cheek from the hand of the Bishop to make him recollect that, as a valiant combatant, he should be prepared to endure with unconquered spirit all adversities for the name of Christ.