حَدَّثَنَا مُحَمَّدُ بْنُ الْمِنْهَالِ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ مَسْلَمَةَ، عَنْ عَمِّهِ، أَنَّ أَسْلَمَ، أَتَتِ النَّبِيَّ صلى الله عليه وسلم فَقَالَ ‏"‏ صُمْتُمْ يَوْمَكُمْ هَذَا ‏"‏ ‏.‏ قَالُوا لاَ ‏.‏ قَالَ ‏"‏ فَأَتِمُّوا بَقِيَّةَ يَوْمِكُمْ وَاقْضُوهُ ‏"‏ ‏.‏ قَالَ أَبُو دَاوُدَ يَعْنِي يَوْمَ عَاشُورَاءَ ‏.‏

Narrated 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. Translation Not Available

Book Ref: Sunan Abi Dawud Book 14 Hadith 2447
Web Ref:  Sunan Abi Dawud Book 13 Hadith 2441