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.  

أَنَّ أَسْلَمَ أَتَتِ النَّبِيَّ ﷺ فَقَالَ صُمْتُمْ يَوْمَكُمْ هَذَا قَالُوا لاَ قَالَ فَأَتِمُّوا بَقِيَّةَ يَوْمِكُمْ وَاقْضُوهُ