Al-Kāfi - The Book of Prayer

Chapter: Recitations in Salah (prayer) 21ـ وَعَنْهُ عَنْ أَحْ٠َانَ بْنِ عِيسَى عَنْ سَ٠َاعَةَ قَالَ سَأَلْتُهُ عَنْ قَوْلِ الله عَزَّ وَجَلَّ وَلا تَجْهَرْ بِصَلاتِكَ وَلا تُخافِتْ بِها قَالَ الْ٠ْعِكَ وَالْجَهْرُ أَنْ تَرْفَعَ صَوْتَكَ شَدِيداً. 21.

It is a narration from him (narrator of previous Hadith) by Ahmad ibn Muhammad from ‘Uthman ibn ‘Isa from Sama’ah who has said the following: “I once asked him (the Imam) (a.s.), about the words of Allah, most Majestic, most Glorious, ‘Do not be loud in your Salah (prayer) nor very quiet.’ (17:110) He (the Imam) said, ‘Quiet is when you do not hear it, and loud is when you raise your voice very loud.’”