Mukhtasar Muslim

Sayyiduna Abu Qatadah (may Allah be pleased with him) says: I went to the mosque and the Messenger of Allah (peace and blessings be upon him) was sitting among the people, so I also sat down. The Messenger of Allah (peace and blessings be upon him) said: Who stopped you from praying two rak‘ahs before sitting down? I replied: O Messenger of Allah! I saw you and the people sitting, so I sat down (as well). The Messenger of Allah (peace and blessings be upon him) said: When any of you enters the mosque, he should not sit until he has prayed two rak‘ahs.