How did the number of Raka'hs come to be? Why do we pray silently in Zuhr and Asr?
Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no doubt that the Muslim Ummah, individuals and groups, have been passing on the number of Raka'hs of the obligatory prayers without differences or disagreement between them. For example, no difference among Muslims that al-Zuhr prayer consists of four Raka'hs (for a resident) and so on, the other prayers.
In this context, al-Shafi'e (may Allah's Mercy be upon him) stated such an issue simply and precisely, saying: 'Allah, The Great and Almighty, has clearly ordained the obligation of prayer in His Book and stated through His Prophet (Sallallahu Alaihi wa Sallam): (1) number of each obligatory prayer, and (2) what should be done and avoided during prayer. Then, the Muslim common people conveyed the number of Rak'ahs of each prayer and there was no need for special reports of scholars who also conveyed the same without a bit of difference. So, they conveyed that al-Zuhr consists of four Raka'hs with low voice in recitation; al-'Asr consists of four Raka'hs with low voice in recitation; al-Maghrib is three Raka'hs with a loud voice only in the first two Raka'hs; al-'Isha is four with aloud voice only in the first two Rak'ahs, and al-Fajr consists of two Raka'hs with a loud voice during recitation'.
Thus, we pray al-Zuhr and al-'Asr with a low voice because the Prophet (Sallallahu Alaihi wa Sallam) did so. He said: "Perform prayers as you have seen me praying" .
Maybe the wisdom behind praying with a loud voice at night and at the time of al-Fajr that could be more beneficial, since people are not busy as they are during the times of al-Zuhr and al-'Asr.
Allah knows best.
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