Musnad Ahmad 25:1305

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Bishr, from Sa'id ibn Jubayr, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) arrived in Medina, and behold, the Jews were fasting the day of Ashura. He asked them about that, and they said: "This is the day on which Moses prevailed over Pharaoh." The Prophet (peace and blessings of Allah be upon him) said to his companions: "You are more entitled to Moses than they are, so fast it."