From Sahih Muslim (Hadith 2669):
Rubayyi' daughter of Mu'awwidh b. 'Afra' said that the Messenger of Allah (ﷺ) sent (a person) on the morning of Ashura to the villages of Ansar around Medina (with this message):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
Reference: Muslim 2669