Musnad Ahmad 1007:13

Sufyan narrated to us, from Uthman ibn Abi Sulayman and Ibn Ajlan, from Amir ibn Abdullah ibn al-Zubayr, from Amr ibn Sulaym, from Abu Qatadah, from the Prophet, peace and blessings of Allah be upon him, who said: "When one of you enters the mosque, let him pray two rak'ahs before he sits down."