Sitting briefly to rest during prayer is Sunnah and mustahabb
Question

Is sitting briefly to rest (jalsat al-istiraahah) when getting up after the first rak’ah to start the second, and when getting up after the third rak’ah to start the fourth, waajib (obligatory) or Sunnah mu’akkadah (a confirmed Sunnah)?
Praise be to Allah.The scholars are agreed that for the worshipper to sit briefly after rising from the second sujood (prostration) of the first or third rak’ah and before standing up to do the following rak’ah is not one of the obligatory parts of prayer, not is it Sunnah mu’akkadah (a confirmed Sunnah). But after that they differed as to whether it is simply Sunnah or it is not one of the obligatory parts of prayer at all, or whether it should be done by those who need to do it because of old age, sickness or being heavy.
 Al-Shafaa’i and a group of hadeeth scholars said: it is Sunnah and it is one of the two opinions narrated from Ahmad, because of the hadeeth narrated by al-Bukhaari and the authors of al-Sunan from Maalik ibn al-Huwayrith, who said that he saw the Prophet (peace and blessings of Allaah be upon him) praying Witr, and he did not stand up until he had first sat up straight. (Narrated by al-Bukhaari in al-Adhaan, 818).
 But the majority of scholars, including Abu Haneefah and Maalik, and Ahmad (may Allaah have mercy on him) according to the second opinion narrated from him, did not think that, because the other ahaadeeth do not mention this sitting. It may be that the sitting which was mentioned in the hadeeth of Maalik ibn al-Huwayrith happened at the end of the Prophet’s life when he had put on weight, or for some other reason.
 A third group of scholars reconciled the ahaadeeth by suggesting that the Prophet (peace and blessings of Allaah be upon him) sat [at this point during the prayer] when he needed to do so.
 I say that it is prescribed to do this when there is a need to do so, but not at other times. It appears that this is mustahabb, and the fact that it is not mentioned in the other ahaadeeth does not indicate that it is not mustahabb, rather it indicates that it is not obligatory.
 The view that it is mustahabb is supported by two things:
 1 – The basic principle concerning the Prophet’s actions is that he did them in order to prescribe them and so that his example may be followed.
 2 – This sitting is proven in the hadeeth of Abu Humayd al-Saa’idi which was narrated by Ahmad and Abu Dawood with a jayyid isnaad, in which he [Abu Humayd] described the prayer of the Prophet (peace and blessings of Allaah be upon him) in the presence of ten of the Sahaabah (may Allaah be pleased with them), and they confirmed that.