Musnad Ahmad 1007:85

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, 'Amr ibn Yahya al-Ansari narrated to us, Muhammad ibn Yahya ibn Habban narrated to us, from 'Amr ibn Sulaym ibn Khaldah al-Ansari, from Abu Qatadah, who said: I entered the mosque while the Messenger of Allah, peace and blessings of Allah be upon him, was sitting among the people, so I sat down. The Messenger of Allah, peace and blessings of Allah be upon him, said: "What prevented you from performing two rak'ahs before you sat down?" I said: "I saw you sitting and the people sitting." He said: "When one of you enters the mosque, let him not sit until he performs two rak'ahs."