وَلَا تَجْهَرْ بِصَلَاتِكَ وَلَا تُخَافِتْ بِهَا
Meaning: Do not make your prayer excessively loud, nor make it excessively quiet.
Quran 17:110 contains the terms jahr and ikhfat; the specific modes of recitation in the obligatory prayers are explained through prophetic practice.