Musannaf Ibn Abi Shayba 3:2109

Ibn ‘Ulayyah narrated to us, from ‘Abd al-‘Aziz ibn Suhayb, that Anas sat in the third rak‘ah. So they glorified Allah (said SubhanAllah), and he stood up and completed it as four. When he performed the taslim, he prostrated two prostrations. Then he turned to the people and said, "If you become confused, then do like this."