Narrated AbdurRahman ibn Maslamah:

AbdurRahman reported on the authority of his uncle that the people of the tribe Aslam came to the Prophet ﷺ. 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.

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