What is the ruling on a mother saying to her daughter: "You will be an unbeliever and not of the religion of Muhammad, peace and blessings be upon him, if you hold this mobile phone again," knowing that the daughter held the phone while forgetting?

Whoever says "I disbelieve in God" or "I am not upon the religion" when intending to do a specific act, and then does it, has committed a great sin. However, this does not constitute disbelief unless one intended to assent to disbelief.

The first [ruling] in this regard is that no expiation (kaffarah) is due, because one does not disbelieve unless they harbor it in their heart. And there is no expiation for breaking an oath upon them, because expiation is only for an oath sworn by God, or by one of His names, or by one of His attributes.

The one who has done this must make sincere repentance, frequently seek forgiveness, perform good deeds, and resolve not to repeat it. As for one who did this out of forgetfulness, there is no violation (ḥinth) upon them and no expiation.