In a dream it is an awakening from heedlessness, and it is a benefit. A woman slapping her own face signifies glad tidings of a male child after having despaired of one, because of His saying, the Exalted:
فَأَوْجَسَ مِنْهُمْ خِيفَةً ۖ قَالُوا۟ لَا تَخَفْ ۖ وَبَشَّرُوهُ بِغُلَـٰمٍ عَلِيمٍ فَأَقْبَلَتِ ٱمْرَأَتُهُۥ فِى صَرَّةٍ فَصَكَّتْ وَجْهَهَا وَقَالَتْ عَجُوزٌ عَقِيمٌ
“And he felt from them apprehension. They said, "Fear not," and gave him good tidings of a learned boy. And his wife approached with a cry [of alarm] and struck her face and said, "[I am] a barren old woman!"”
Quran 51:28–29 · Saheeh International
Slapping is injustice on the part of the one who slaps toward someone who did him no wrong. Slapping on the cheek or elsewhere on the body may signify illnesses and calamities in the limb that was slapped.
Whoever sees a person slap him, or that he slapped a person, will rouse him from heedlessness.