Sahih Bukhari Volume 3, Book 30, Hadith 1961

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، حَدَّثَنَا خَالِدُ بْنُ ذَكْوَانَ، عَنِ الرُّبَيِّعِ بِنْتِ مُعَوِّذٍ، قَالَتْ أَرْسَلَ النَّبِيُّ صلى الله عليه وسلم غَدَاةَ عَاشُورَاءَ إِلَى قُرَى الأَنْصَارِ ‏"‏ مَنْ أَصْبَحَ مُفْطِرًا فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ، وَمَنْ أَصْبَحَ صَائِمًا فَلْيَصُمْ ‏"‏‏.‏ قَالَتْ فَكُنَّا نَصُومُهُ بَعْدُ، وَنُصَوِّمُ صِبْيَانَنَا، وَنَجْعَلُ لَهُمُ اللُّعْبَةَ مِنَ الْعِهْنِ، فَإِذَا بَكَى أَحَدُهُمْ عَلَى الطَّعَامِ أَعْطَيْنَاهُ ذَاكَ، حَتَّى يَكُونَ عِنْدَ الإِفْطَارِ‏

Narrated By Ar-Rubi' bint Mu'awadh : "The Prophet sent a messenger to the village of the Ansar in the morning of the day of 'Ashura (10th of Muharram) to announce: 'Whoever has eaten something should not eat but complete the fast, and whoever is observing the fast should complete it.' "She further said, "Since then we used to fast on that day regularly and also make our boys fast. We used to make toys of wool for the boys and if anyone of them cried for, he was given those toys till it was the time of the breaking of the fast."