Abu Dawud
Fasting
Book Number: 007 / Hadith Number: 2441
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.