Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.111D.

Allah says: “Neither say your prayer aloud, nor speak it in a low tone, but seek a middle course between.” Qur’an 17.110 And “Call on your Lord with humility and in private, for Allah does not love those who go beyond bounds.” Qur’an 7.55

Abu Musa Al-Ash’ari reported, “Once the people raised their voices in supplication. At this the Prophet, peace be upon him, said, ‘Be easy on yourselves. You are not calling upon someone deaf or absent. You are calling upon one Who is All-seeing and All-hearing. He is nearer to you than the neck of your mount. O Abdallah b. Qais, shall I tell you a word that is one of the treasures of Paradise? It is: “There is no power nor any might except with the permission of Allah”.’ (Muslim and Bukhari)

Abdallah b. ‘Umar reported that the Prophet, peace be upon him, said, “Hearts are like vessels, some more attentive and capacious than others. When you supplicate Allah you should be certain of being answered, and know that Allah does not answer a supplication that comes from a careless and inattentive heart.” (Ahmad)