Abu Dawud Book 2, Hadith Number 575.
Chapter : Not known.
Narated By Yazid ibn al-Aswad : Yazid prayed along with the Apostle of Allah (PBUH) when he was a young boy. When he (the Prophet) had prayed there were two persons (sitting) in the corner of the mosque; they did not pray (along with the Prophet). He called for them. They were brought trembling (before him). He asked: What prevented you from praying along with us? They replied: We have already prayed in our houses. He said: Do not do so. If any of you prays in his house and finds that the imam has not prayed, he should pray along with him; and that will be a supererogatory prayer for him.