Musnad Ahmad 1007:62

'Abd al-Rahman ibn Mahdi and 'Abd al-Razzaq narrated to us, they said: Malik narrated to us, from 'Amir ibn 'Abd Allah ibn al-Zubayr, from 'Amr ibn Sulaym, from Abu Qatadah, who said—'Abd al-Razzaq said in his narration: I heard Abu Qatadah say: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you enters the mosque, let him pray two rak'ahs before he sits down."