Fasting (Kitab Al-Siyam)
Dawud :: Book 13 : Hadith 2441
Narrated AbdurRahman ibn Maslamah:
AbdurRahman reported on the authority of his uncle that the people of the tribe Aslam came to the Prophet (peace_be_upon_him). 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.