Wa `alaykum salaam wa rahmatullaah
Jalsatul Istiraahah: This is when a person sits (for a short while) after the second prostration in the 1st Rak`ah or 3rd Rak`ah (Apart from Maghrib). This is done just before he stands up.
The scholars differed as to whether it is Mustahabb to do this or not.
From the scholars who held that it is Mustahabb were Imaam al-Shaafi`ee (this is the known report from him) and from the contemporary scholars, Shaykh `Abdul `Azeez ibn Baaz and Shaykh al-Albaanee held this view.
Generally their evidence is based on the narration of Imaam al-Bukhaari narrated on the authority of Maalik al-Huwayrith, radiyAllaahu `anhu
that he saw the Prophet :saw2: praying and when he had prayed an odd number (of Rak`ah), he did not stand up till he had sat up properly
[Please note the Sahaabi: Maalik al-Huwayrith]
The second opinion is that Jalsatul Istiraahah is not Mustahabb and this was the opinion of the majority of scholars as mentioned by Imaam al-Nawawi and this includes Imaam Abu Haneefah, Imaam Maalik and this was also the known opinion of the Hanaabilah.
The scholars disagreed because it was authentically reported on Ibn `Abbaas, Ibn `Umar, Ibn Mas`ood (as reported by Ibn Abi Shaybah and Ibn Mundhir) that they did not do this (i.e jalsatul istiraahah). And these were sahaabah who were known to follow the Prophet :saw2: very closely.
The scholars mentioned that Maalik Ibn Al Huwayrith came to madeenah in the 9th year after Hijrah, just before the Prophet :saw2: was getting ready for the battle of Tabook and it was narrated that the Prophet became heavier and old. This is why Ibn Taymiyyah rahimahullaah said that it is sunnah for a person to
not do Jalsatul Istiraahah if he is healthy and he is able to stand up straight from the prostration, and he said that it is sunnah to do the Jalsatul Istiraahah if the person is old and is finding it difficult to stand up straight after the prostration. And this is what seems to be the most correct opinion and Allaah knows best.
It should be noted that if the Prophet :saw2: did the Jalsatul Istiraahah very often then it would have been narrated from a large number of companions, rather it was only narrated by Maalik al-Huwayrith, radiyAllaahu `anhu, who only spent 20 days with the Prophet :saw2:... this is what was mentioned by Ibn Qayyim, Al-Sa`dee, Shaykh Muhammad ibn Ibraaheem aal al-Shaykh.
If you did not understand any point, please let me know so that I can clarify inshaa'Allaah.
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Whatever the case, if a person truly believes that Jalsatul Istiraahah is Mustahabb and follows the opinion of Shaykh al-Albaanee or Ibn Baaz then its not a problem. As long as we all appreciate valid differences of opinion, there will be a beautiful brotherhood amongst muslims