The Prophet ﷺ ordered a man from the tribe of Bani Aslam to announce amongst the people that whoever had eaten should fast the rest of the day, and whoever had not eaten should continue his fast, as that day was the day of 'Ashura' .  

أَمَرَ النَّبِيُّ ﷺ رَجُلاً مِنْ أَسْلَمَ أَنْ أَذِّنْ فِي النَّاسِ أَنَّ مَنْ كَانَ أَكَلَ فَلْيَصُمْ بَقِيَّةَ يَوْمِهِ وَمَنْ لَمْ يَكُنْ أَكَلَ فَلْيَصُمْ فَإِنَّ الْيَوْمَ يَوْمُ عَاشُورَاءَ  

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  

[Machine] The Prophet (pbuh) sent a man on the day of Ashura who embraced Islam saying, "Indeed, today is the day of Ashura, so whoever has eaten or drank should complete the rest of his day, and whoever has not eaten or drank should observe fasting."  

أَنَّ النَّبِيَّ ﷺ بَعَثَ يَوْمَ عَاشُورَاءَ رَجُلًا مِنْ أَسْلَمَ «إِنَّ الْيَوْمَ يَوْمُ عَاشُورَاءَ فَمَنْ كَانَ أَكَلَ أَوْ شَرِبَ فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ وَمَنْ لَمْ يَكُنْ أَكَلَ أَوْ شَرِبَ فَلْيَصُمْهُ»  

[Machine] The Prophet ﷺ said to Aslam, "Fast today." They said, "But we have already eaten." He said, "Then fast the rest of your day, meaning the day of Ashura." Sa'id disagreed with him.  

أَنَّ النَّبِيَّ ﷺ قَالَ لِأَسْلَمَ «صُومُوا الْيَوْمَ» قَالُوا إِنَّا كُنَّا قَدْ أَكَلْنَا قَالَ «صُومُوا بَقِيَّةَ يَوْمِكُمْ يَعْنِي يَوْمَ عَاشُورَاءَ» خَالَفَهُ سَعِيدٌ  

He who got up in the morning fasting (without eating anything) he should complete his fast, and he who had had his breakfast in the morning, he should complete the rest of the day (without food). The Companions said; We henceforth observed fast on it (on the day of 'Ashura) and, God willing, made our children observe that. We went to the mosque and made toys out of wool for them and when anyone felt hungry and wept for food we gave them these toys till it was the time to break the fast. (Using translation from Muslim 1136a)   

أَرْسَلَ رَسُولُ اللهِ ﷺ صَبِيحَةَ عَاشُورَاءَ إِلَى قُرَى الْأَنْصَارِ الَّتِي حَوْلَ الْمَدِينَةِ مَنْ كَانَ أَصْبَحَ صَائِمًا فَلْيُتِمَّ صَوْمَهُ وَمَنْ كَانَ أَصْبَحَ مُفْطِرًا فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ قَالَتْ فَكُنَّا نَصُومُهُ بَعْدَ ذَلِكَ وَنُصَوِّمُ صِبْيَانَنَا الصِّغَارَ وَنَجْعَلُ لَهُمُ اللُّعْبَةَ مِنَ الْعِهْنِ وَنَذْهَبُ بِهِمْ إِلَى الْمَسْجِدِ فَإِذَا بَكَى أَحَدُهُمْ عَلَى الطَّعَامِ أَعْطَيْنَاهُ ذَلِكَ حَتَّى يَكُونَ عِنْدَ الْإِفْطَارِ  

[Machine] The Messenger of Allah ﷺ sent a man from among the Muslims to his people on the day of Ashura and said: "Command them to observe fasting on this day." The man replied: "O Messenger of Allah, I fear that they will not adhere to it until they eat." He ﷺ said: "Whoever among them eats, let him fast the rest of the day."  

أَنَّ رَسُولَ اللهِ ﷺ بَعَثَ رَجُلًا مِنْ أَسْلَمَ إِلَى قَوْمِهِ يَوْمَ عَاشُورَاءَ فَقَالَ مُرْهُمْ فَلْيَصُومُوا هَذَا الْيَوْمَ فَقَالَ يَا رَسُولَ اللهِ مَا أَرَانِي آتِيهِمْ حَتَّى يَطْعَمُوا قَالَ مَنْ طَعِمَ مِنْهُمْ فَلْيَصُمْ بَقِيَّةَ يَوْمِهِ  

The Prophet ﷺ ordered a man from the tribe of Bani Aslam to announce amongst the people that whoever had eaten should fast the rest of the day, and whoever had not eaten should continue his fast, as that day was the day of 'Ashura' . (Using translation from Bukhārī 2007)   

"أَذِّنْ في النَّاسِ: أَنَّ مَنْ كان أكَلَ فليَصُمْ بقيَّةَ يوْمِه، ومَنْ لم يكنْ أَكَلَ فليَصُمْ، فإِنَّ اليَوْمَ يومُ عاشُوراءَ".