Musnad Abu Ya'la al-Mawsili 109:118

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Suhayl ibn Abi Salih, from Amir ibn Abdullah ibn al-Zubayr, from Amr ibn Sulaym al-Zuraqi, from Jabir, that the Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you enters the mosque, let him pray two rak'ahs before he sits."