When the Prophet (ﷺ) came to Medina, he found (the Jews) fasting on the day of 'Ashura' (i.e. 10th ofMuharram). They used to say: "This is a great day on which Allah saved Moses and drowned the folkof Pharaoh. Moses observed the fast on this day, as a sign of gratitude to Allah." The Prophet (ﷺ) said, "Iam closer to Moses than they." So, he observed the fast (on that day) and ordered the Muslims to faston it.