Prophet salllahu alaihi wasalam said, “When you prostrate, prostrate firmly, then when you rise, sit on your left thigh” (Ahmad)
There are two ways as according to sunnah, to perform sitting between two prostrations
With regard to the way one should sit between sujoods, two ways of doing this have been narrated from the Prophet (peace and blessings of Allaah be upon him).
Keeping the right foot upright and sitting on the left foot, as was described in the saheeh hadeeth which describes how he sat between two sujoods: “He kept his right foot upright,” and “he pointed its toes towards the qiblah.” This is the most well known way, and it is the way he sat most of the time. This way of sitting is called MUFTARISHAN.
Keeping both feet upright and sitting on the heels, as was described in the saheeh hadeeth which describes how he sat between two sujoods: “Sometimes he would rest on both his heels and the balls on his feet.” This way of sitting is called IQ’A
Picture taken from HERE
The Obligation of being at Ease between the two Sajdahs
Prophet Muhammad sallallahu alaihi wasalam would be relaxed until his bone returned to its proper position when he sits in between the sajdahs. He ordered those who prayed badly to do likewise.
“He (sallallaahu ‘alaihi wa sallam) would be relaxed until every bone returned to its (proper) position”(Abu Daawood & Baihaqi with a saheeh isnaa)d., and he ordered “the one who prayed badly” likewise, and said to him, The prayer of any of you is not complete until he does this.( Abu Daawood & Haakim , who declared it saheeh and Dhahabi agreed.)
Lengthening the Sitting between the two Sajdahs
Prophet Muhamamd sallalahu alaihi wasalam would lenghthen his sitting until it was almost as long as his sajdah (Bukhaari & Muslim)., and sometimes, he would remain in his position until one would think: He has forgotten.
A comment by Ibn Qayyim on this matter : Ibn al-Qayyim said, “This sunnah was abandoned by the people after the time of the Companions. But as for the one who abides by the Sunnah, and does not glance sideways towards whatever contradicts it, he is unworried by anything opposing this guidance.”
O Allaah! (in one version: O my Lord!) Forgive me; have mercy on me; [strengthen me;] [raise my rank;] guide me; [pardon me;] sustain me (Abu Daawood, Tirmidhi, Ibn Maajah & Haakim , who declared it saheeh and Dhahabi agreed.)
O my Lord! Forgive me, forgive me.(Ibn Maajah )
Ishaaq bin Raahawaih said, “If he wishes, he can say this three times, or he can say O Allaah! Forgive me …, because both of them have been reported from the Prophet (sallallaahu `alaihi wa sallam) between the two sajdahs.”