Musannaf Ibn Abi Shayba 3:2120

Abu Khalid al-Ahmar narrated to us, from Ibn 'Awn, who said: I said to al-Sha'bi, "I prayed two rak'ahs, and when I intended to sit [for the tashahhud], I stood up [instead]." He said, "If it were me, I would have continued."