Praise be to Allah.
It is mustahabb to have the habit of sitting for a brief rest during prayer, i.e., after the two prostrations of each rak’ah in which one does not recite the Tashahhud immediately after the prostrations. A hadeeth to this effect was reported in Saheeh al-Bukhaari. It was also reported in Sunan Abi Dawood and Sunan al-Tirmidhi via different isnaads that are also saheeh. This is the correct view according to scholarly consensus in the madhhab of al-Shaafa’i. Jalsah al-istiraahah is not mustahaab after sajdat al-tilaawah (prostration during recitation of Qur’aan) during prayer. (Words of Imaam al-Nawawi).
Jalsah al-istiraahah is a brief pause, sitting for a short while after the second prostration and before standing up. It is Sunnah according to al-Shaafa’i, Ishaaq and Ahmad. It was reported that Abu Qulaabah (may Allaah be pleased with him) said: “Maalik ibn al-Huwayrith showed us how the Messenger of Allaah (peace and blessings of Allaah be upon him) prayed, and when he raised his head from the second prostration in the first rak’ah, he sat and then stood up.” (Reported by all five major scholars of hadeeth apart from Muslim). The wording according to al-Bukhaari was: “When he raised his head from the second prostration, he sat and settled on the ground, then he stood up.”
I say: one of the features of the prayer is jalsah al-istiraahah which should be done after every prostration from which one stands up.
It is not Sunnah to do Jalsah al-Istiraahah after Sajdat al-Tilaawah, or when one is praying sitting down, or in the fourth or second rak’ah of Zuhr, for example, if a person wants to recite Tashahhud. If he does not want to recite it, then it is Sunnah to do Jalsah al-istiraahah.
It is a separation between the two rak’ahs, according to the soundest opinion, it is not a part of either the first rak’ah or the second.
The best is not to make it last longer than a few seconds, because according to the Sunnah, the shorter it is the better, like when one keeps quiet in prayer.