I came to Madinah and said to myself that I would observe how the Messenger of Allah ﷺ prayed. When he sat for the tashahhud, he spread out his left leg, placed his left hand on his left thigh, and kept his right leg upright.
حدثنا أبو كريب حدثنا عبد الله بن إدريس حدثنا عاصم بن كليب الجرمي عن أبيه
قدمت المدينة قلت لأنظرن إلى صلاة رسول الله ﷺ فلما جلس يعني للتشهد افترش رجله اليسرى ووضع يده اليسرى يعني على فخذه اليسرى ونصب رجله اليمنى
Sourcing, its chain of transmission is sahih. Abu Dawud narrated it in the Book of Prayer, chapter on how to sit, no. 957, and Nasai, 3/34-35, no. 1264, from the hadith of Asim bin Kulayb, with the same content, at greater length, and Ibn Khuzayma graded it sahih, no. 690.