لَا يَحِلُّ لِمُسْلِمٍ أَنْ يُرَوِّعَ مُسْلِمًا
It is not permissible for a Muslim to frighten another Muslim.
Source: أبو داود
لَا يَحِلُّ means “it is not permissible,” and يُرَوِّعَ means to terrify or alarm someone. The same word appears in two grammatical positions: لِمُسْلِمٍ follows the preposition lām and takes kasrah; مُسْلِمًا is the object of the verb and takes fatḥah.
The teaching protects another person’s sense of safety. Frightening someone for entertainment may cause real harm even if the perpetrator later claims it was “only a joke.”