Narrated Ibn `Abbas: When the Prophet came to Medina, he found (the Jews) fasting on the day of 'Ashura' (i.e. 10th of Muharram). They used to say: This is a great day on which Allah saved Moses and drowned the folk of Pharaoh. Moses observed the fast on this day, as a sign of gratitude to Allah. The Prophet said, I am closer to Moses than they. So, he observed the fast (on that day) and ordered the Muslims to fast on it.