This hadith describes the manner in which the Prophet ﷺ sat between the two prostrations and also during the first tashahhud (in prayers of three or four rak‘ahs).
This sitting posture is called “Iftirash.”
In this position, the worshipper:
Places the left foot under the body and sits upon it.
Keeps the right foot upright, with the toes pointing toward the qiblah.
حدثنا إسحق بن إبراهيم أخبرنا محمد بن بكر قال وحدثنا حسن الحلواني حدثنا عبد الرزاق وتقاربا في اللفظ قالا جميعا أخبرنا ابن جريج أخبرني أبو الزبير أنه سمع طاوسا يقول قلنا لابن عباس في الإقعاء على القدمين فقال هي السنة فقلنا له إنا لنراه جفاء بالرجل فقال ابن عباس بل هي سنة نبيك صلى الله عليه وسلم
Ishaq ibn Ibrahim narrated to us, Muhammad ibn Bakr informed us, and Hasan al-Halwani also narrated, saying: ‘Abd al-Razzaq narrated to us; their wordings were similar. Both said: Ibn Jurayj informed us, saying: Abu az-Zubayr informed me that he heard Tawus say:
We said to Ibn ‘Abbas about sitting in iq‘a’ (sitting on the heels). He said: “It is the Sunnah.”
We said to him: “We see it as rough or improper for a man.”
Ibn ‘Abbas replied: “Rather, it is the Sunnah of your Prophet ﷺ.”
(Sahih Muslim, Book of Prayer, Chapter on Sitting Between the Two Prostrations, Hadith 536)
Therefore, sitting on the heels (iq‘a’) between the two prostrations is a Sunnah practice that the Prophet ﷺ performed. However, most often he would sit in the iftirash position—sitting on the left foot with the right foot upright.