Musnad Ahmad 1007:136

Waki’ narrated to us, from Abu al-Umays, from ‘Amir ibn ‘Abd Allah ibn al-Zubayr, from al-Zurqi, from Abu Qatadah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you enters the mosque, let him not sit down until he prays two rak'ahs."