Keep your voice low in Allah's house. ʿUmar rebuked two men who raised their voices in the Prophet's masjid, saying: “Had you been from this city, I would have punished you — raising your voices in the masjid of the Messenger of Allah ﷺ!” (al-Bukhārī). Loud voices disturb its calm and others' khushūʿ.
العربية: عن السائب بن يزيد رضي الله عنه قال: كنتُ قائمًا في المسجد فحصبني رجلٌ، فنظرتُ فإذا عمر بن الخطاب، فقال: اذهب فأتني بهذين… ثم قال: لو كنتما من أهل البلد لأوجعتكما، ترفعان أصواتكما في مسجد رسول الله ﷺ. (البخاري).
In the masjid, speak softly — preserve its tranquillity and the worshippers' focus.