Abu Dawud Book 2, Hadith Number 577.

Chapter : Not known.

Narated By Yazid ibn Amir : I came while the Prophet (PBUH) was saying the prayer. I sat down and did not pray along with them. The Apostle of Allah (PBUH) turned towards us and saw that Yazid was sitting there. He said: Did you not embrace Islam, Yazid? He replied: Why not, Apostle of Allah; I have embraced Islam. He said: What prevented you from saying prayer along with the people? He replied: I have already prayed in my house, and I thought that you had prayed (in congregation). He said: When you come to pray (in the mosque) and find the people praying, then you should pray along with them, though you have already prayed. This will be a supererogatory prayer for you and that will be counted as obligatory.