Musannaf Ibn Abi Shayba 3:4036

'Abbad ibn al-'Awwam narrated to us, from Husayn, from al-Qasim ibn Abi Ayyub, from Sa'id ibn Jubayr, that he came to the mosque while the Imam was in the dawn prayer. So he prayed the two rak'ahs before he entered the mosque, at the door of the mosque.