Musnad Ahmad 1007:7

'Abd al-Rahman ibn Mahdi narrated to us, Malik—meaning ibn Anas—narrated to us, from 'Amir ibn 'Abd Allah—meaning ibn al-Zubayr—from 'Amr ibn Sulaym, from Abu Qatadah, who said: 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."