He saw the Prophet ﷺ sitting in prayer with his left leg folded beneath him and his forearms resting on his thighs, pointing with his index finger as he supplicated.
أخبرنا محمد بن علي بن ميمون الرقي قال حدثنا محمد وهو ابن يوسف الفريابي قال حدثنا سفيان عن عاصم بن كليب عن أبيه
أنه رأى النبي ﷺ جلس في الصلاة فافترش رجله اليسرى ووضع ذراعيه على فخذيه وأشار بالسبابة يدعو بها
[Sahih] Reported by Abu Dawud, Prayer, Chapter: How to sit in the tashahhud, no. 957, from the hadith of Asim at length. Tirmidhi said, no. 292, hasan sahih. See also the previous hadith, this is an excerpt from it, and it is in Al-Kubra, no. 1187.