Abu Dawud Book 7, Hadith Number 2441.
Chapter : Not known.
Narated By AbdurRahman ibn Maslamah : AbdurRahman reported on the authority of his uncle that the people of the tribe Aslam came to the Prophet (PBUH). He said (to them): Did you fast on this day? They replied: No. He said: Complete the rest of your day, and make atonement for it.