All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
It is authentically confirmed that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever prays four Rak’ahs before Thuhr and four Rak’ahs after Thuhr, Allaah will forbid Hellfire from him.” [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah]
The matter is not, as you think, that these eight Rak’ahs are separate from the Rawaatib [regular voluntary prayers] before and after the Thuhr prayer; rather, these eight Rak’ahs include the Rawaatib of Thuhr prayer.
Therefore, you would pray four Rak’ahs of the Rawaatib before Thuhr as usual; and after Thuhr, you would pray two Rak’ahs of the Rawaatib as usual and then two more Rak’ahs.
In that case, you would have prayed four Rak’ahs before Thuhr and four Rak'ahs after Thuhr; six of which are the Rawaatib of Thuhr, and two additional Rak’ahs.
Ibn Baaz said in one of his Fatwas: “If a person regularly prays four Rak’ahs before Thuhr prayer and four Rak’ahs after Thuhr, Allaah will forbid the Hellfire from him.” It was reported by Ahmad, Abu Daawood, At-Tirmithi, An-Nassaa’i and Ibn Maajah, with a correct chain of narrators on the authority of Umm Habeebah
. It means that the person prays two extra Rak'ahs after the Sunnah Raatibah [regular voluntary prayers] after the Thuhr prayer, as the Sunnah Raatibah are four Rak’ahs before it and two Rak’ahs after it, and so, if he prays the two Rak’ahs after [the Rawaatib], then what was mentioned in the Hadeeth of Umm Habeebah
is achieved.”
Asna Al-Mataalib – a Shaafi'i book – reads: "There are ten confirmed Rawaatib of obligatory prayers, and the wisdom behind them is to compensate for the deficiency in obligatory prayers – from the Mercy and Grace of Allaah. They are as follows: two Rak’ahs before Fajr, two Rak’ahs before Thuhr, two Rak’ahs after Thuhr, two Rak’ahs after Maghrib, two Rak’ahs after ‘Ishaa’… and it is a Sunnah to pray an additional two Rak’ahs before Thuhr … and two Rak’ahs after it, for the Hadeeth which reads “Whoever prays four Rak’ahs before Thuhr prayer and four Rak’ahs after Thuhr, Allaah will forbid the Hellfire from him.” [reported by At-Tirmithi and others]”
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Allaah Knows best.