What is the ruling on a woman who said to her husband in a moment of anger, "I have committed an act of disbelief," and slapped her face, and is expiation due upon her?

If a woman utters a word of disbelief while she is sane and chooses to do so, and her anger does not lead her out of her senses, then she has apostatized from Islam. This is based on the saying of Ibn Taymiyyah: "Whoever utters a word of disbelief without necessity, doing so intentionally and knowing that it is a word of disbelief, then he disbelieves by it outwardly and inwardly." Scholars have agreed on deeming as a disbeliever anyone from whom a statement of disbelief emanates, whether it was said in jest, out of stubbornness, or out of conviction. In this situation, what is incumbent upon her is sincere repentance, renewing her faith, and increasing in righteous deeds.

However, if she uttered such a statement during intense anger without intending the word, then there is nothing upon her. Slapping the cheeks is an act of Jaahiliyyah (the pre-Islamic era of ignorance).