Musannaf Ibn Abi Shayba 3:1042

Abu Bakr narrated to us, he said: Yahya ibn Sa'id al-Qattan narrated to us, from Ibn 'Ajlan, from 'Amir ibn 'Abd Allah ibn al-Zubayr, from 'Amr ibn Sulaym, from Abu Qatadah, that the Prophet, peace and blessings of Allah be upon him, said: "When you enter the mosque, then pray two rak'ahs before you sit down."