"When the Prophet (ﷺ) would sit in the prayer, he would place his left foot horizontally - so much so that the upper-part of his foot became black."
Sunan Abi Dawud Book 2, Hadith 573
← Previous Hadith (There is another chain) from Yahya bin Sa'eed that Al-Qasim bin Muhammad saw them sitting in Tashah-hud, so he mentioned the Hadith.

Sunan Abi Dawud Book 2, Hadith 572 Next Hadith → I am more informed than any of you regarding the manner in which the Messenger of Allah (ﷺ) offered his prayer. They said: Present it. The narrator then reported the tradition, saying: he bent the toes of his feet turning them towards the Qiblah when he prostrated, then he uttered “ Allah is most great,” and raised (his head), and bent his left foot and sat on it, and he did the same in the second Rakah. The narrator then transmitted the tradition, and added: In the prostration (i.e., the Rakah) which ended at the salutation, he sat on the hips at the left side. ahmad (b. Hanbal) added: they said : You are right. This is how he used to pray. They (Ahmed and Musaddad) did not mention in their versions how he sat after offering two rak’ahs of prayer.

Sunan Abi Dawud Book 2, Hadith 574