Imam al-Shafi'i (may Allah have mercy on him) said: "Indeed Allah, the Mighty and Majestic, says: 'And do not recite your prayer too loudly, nor too quietly.' [Surah al-Isra: 110]
What is meant by it (the prayer) is supplication, and Allah knows best.
And the meaning of 'Do not be loud' is that you should not raise your voice very high, and the meaning of 'nor too quiet' is that it should not be so quiet that you yourself cannot hear it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q3008
Concerning the saying of Allah Almighty, 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110], it is reported from Sayyidah Aishah (may Allah be pleased with her) that what is meant by it is supplication.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3008
Sayyidah Aishah (may Allah be pleased with her) says that this verse, 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110], was revealed concerning supplication.
(b) Likewise, Mujahid said that it was revealed concerning asking in supplication.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3009
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), concerning this saying of Allah Almighty, 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110], says: When a man made supplication in prayer, he would raise his voice.
So it is in this narration, and it is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3010
It is reported from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the verse 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110] came down when the Messenger of Allah (peace and blessings be upon him) was in hiding in Makkah. When he (peace and blessings be upon him) raised his voice in prayer, the idolaters would hear it and would revile the Qur'an, the One who revealed it and the one who brought it. So Allah Almighty said: 'And do not recite your prayer too loudly' [Surah al-Isra: 110], "Do not raise your voice in your prayer so much that the idolaters hear," 'nor too quietly' [Surah al-Isra: 110], "and do not recite it so softly that even your companions cannot hear," 'but seek a way between that' [Surah al-Isra: 110], "rather, adopt a moderate way: let them hear you, but do not raise your voice too loud, so that they may take the Qur'an from you."
(b) It is also possible that all of these matters are meant by this verse.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3011
Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) says: When the Messenger of Allah (peace and blessings be upon him) set out for the Battle of Hunayn, the people climbed onto the mounds and began saying the takbir in very loud voices. The Messenger of Allah (peace and blessings be upon him) said: "O people, recite your glorifications and takbirs softly; you are not calling upon one who is deaf or absent, rather you are calling upon One who is All-Hearing and Near, and He is with you." The narrator says: I was behind the Messenger of Allah (peace and blessings be upon him) and was saying: 'La hawla wa la quwwata illa billah' (There is no power and no strength except with Allah). He (peace and blessings be upon him) said: "O Abdullah bin Qays!" I said: Here I am, O Messenger of Allah! He (peace and blessings be upon him) said: "Shall I not tell you of a word that is from the treasures of Paradise?" I said: Of course, please do tell me! May my father and mother be ransomed for you. He (peace and blessings be upon him) said: "'La hawla wa la quwwata illa billah' is from the treasures of Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3012