Prophetic Guideline on Quiet and Loud recitation in Salaah
Shaykh al-Albaanee (may Allaah have mercy on him) said:
He (sallallaahu 'alayhi wasallam) used to recite loudly in the morning prayer and in the first two rak’ah of Maghrib and ‘Ishaa, and quietly in Zuhr, ‘Asr, the third rak’ah of Maghrib and the last two rak’ah of ‘Ishaa. They [the Companions] could tell when he was reciting quietly from the movement of his beard.[1]
Shaykh Muhammad bin ‘Umar bin Saalim Baazmool commented:
The scholars said: The guideline with regard to loud recitation is that a person should recite the aayah of the Qur’aan in a manner that the one by his side would hear him. Therefore, if a person recites the Qur’aan and the one by his side hears him, he has indeed recited a loud recitation. The scholars also said: As for quiet recitation, it is for a person to recite the Qur’aan and the adhkaar by moving his tongue in a manner that he can hear himself while the person close to him will not hear him. It is essential for the tongue to move in a quiet recitation. Shaykh al- Islaam was of the view that it is valid even if he cannot hear himself. However, other scholars are of the view that it is essential to move the tongue in quiet recitation and for the person to hear himself in such a way that if anyone were to place his ear close to his head, that person would know what the reciter is reciting in quiet prayer.
Having known the guideline of quiet and loud recitation [in Salaah], we say: what some people do whereby they would stand up for prayer with closed lips without moving their tongue till they finish the prayer - they do not move their tongue with recitation of the Qur’an while standing, nor move their tongue while reciting the adhkaar in rukoo and while rising from it, nor in sujood and while rising from it, nor while sitting for tashahhud; the prayer of these people is invalid; because they did not recite anything in the Salaah. It is a must that the tongue should move in recitation. According to some scholars, it is essential for the tongue to move in quiet prayers and the worshipper should hear himself. This is an important issue.
Many times people would come and say: we regularly recite the morning and evening adhkaar, and seek refuge with Allaah, in spite of that we were afflicted with such and such. We say: If you say the morning and evening adhkaar quietly, it is necessary for you to move your tongue. Passing your eyes along the lines of the page will be of no benefit. This cannot be called recitation or speech in Arabic language. In Arabic language, it is essential for the tongue to move in recitation. That is why it is recorded in the hadeeth that the Companions used to recognise the recitation of the Messenger (sallallaahu alayhi sallam) in quiet prayers from the movement of his beard which indicates that the Prophet (sallallaahu alayhi wasallam) used to move his tongue and lips even in quiet Salaah.
Source: Sharhu Kitaabi Sifati Salaah an-Nabiyy min at-Takbeer Ilaa at-Tasleem Ka’annaka Taraaha of Shaykh al-Albaanee, p. 200-201 by Shaykh Muhammad bin ‘Umar bin Saalim Baazmool
[1] Aboo Ma’mar narrated: I asked Khabbab whether the Prophet used to recite the Qur'an in the Zuhr and the 'Asr prayers. He replied in the affirmative. We said, "How did you come to know that?" He said, "From the movement of his beard." Recorded by al-Bukhaaree

JAZAKUM ALLAHU KHEARAN MAY ALMIGHTY ALLAH INCREASE US IN KNOWLEDGE.