Sahih Muslim Book 4, Hadith Number 1541.

Abu Qatada, a Companion of the Messenger of Allah (may peace be upon him), said: I entered the mosque, when the Messenger of Allah (may peace be upon him) had been sitting among people, and I also sat down among them. Upon this the Messenger of Allah (may peace be upon him) said: What prevented you from offering two rak’ahs (of Nafl prayer) before sitting down? I said: Messenger of Allah, I saw you sitting and people sitting (around you and I, therefore, sat in your company). He (the Holy Prophet) then said: When anyone among you enters the mosque, he should not sit till he has observed two rak’ahs.