Musannaf Ibn Abi Shayba 3:6337

Ja'far ibn Burqan narrated to us, from 'Uqbah ibn Nafi', who said: I heard Ibn 'Umar saying: There is no prayer except that there is in it recitation, and sitting in the two rak'ahs, and the tashahhud, and the taslim. If you do not do that, you prostrate two prostrations after you have performed the taslim, while you are sitting.