The Big Dictionary of Dreams

by Martha Clarke
A slap always indicates a drastic reaction. It can be interpreted as a warning from the subconscious to rectify a judgment or an attitude. Maybe, the time is right to take a break along the way and reconsider. It may also warn against an illness that you have not yet noticed any symptoms of.

Islamic Dream Interpretation

by Ibn Seerin
(Striking) Slapping someone on his cheek in a dream means cautioning him or warning him about his heedlessness.

If a woman gently slaps her own cheeks in a dream, it means glad tiding of a son she will conceive at an advanced age, or after havinglost hope in conceivingchildren. However, slappingsomeone’s face in a dream also could mean injustice, an illness, or cold.

(Also see Slapping on the cheek)