Mishkat al-Masabih, hadith 801

وَعَن ابْن عمر والبياضي قَالَا: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِنَّ الْمُصَلِّيَ يُنَاجِي رَبَّهُ فَلْيَنْظُرْ مَا يُنَاجِيهِ بِهِ وَلَا يَجْهَرْ بَعْضُكُمْ عَلَى بَعْضٍ بِالْقُرْآنِ» . رَوَاهُ أَحْمد

Ibn ‘Umar and al-Bayadi reported God’s Messenger as saying, “He who prays holds intimate converse with his Lord, so he must consider how he does so, and none of you must recite the Qur’an more loudly than others. Ahmad transmitted it.