In the loud prayers, listen silently to the imam's recitation — do not recite aloud with him. The Prophet ﷺ finished a prayer and asked: “Did any of you recite with me just now?” A man said yes; he said: “I was wondering why the Qur'an was being contended with me.” So the people stopped reciting aloud behind him. (Tirmidhī — ṣaḥīḥ).
العربية: عن أبي هريرة رضي الله عنه أنّ رسول الله ﷺ انصرف من صلاةٍ جهر فيها بالقراءة فقال: «هل قرأ معي أحدٌ منكم آنفًا؟»… فانتهى الناس عن القراءة معه فيما جهر فيه. (الترمذي).
When the imam recites aloud, listen attentively and quietly — a sign of khushūʿ.