Musnad Ahmad 30:1596

Abu Ja'far narrated to us, Abd al-Samad informed us, from his father, from Shubayl, from Abu Hurayrah, who said: The Prophet, peace and blessings of Allah be upon him, passed by some people of the Jews who had fasted the day of Ashura. He said: "What is this fasting?" They said: "This is the day on which Allah saved Moses and the Children of Israel from drowning, and in which Pharaoh was drowned. And this is the day on which the Ark settled upon Mount Judi, so Noah and Moses fasted it in gratitude to Allah." The Prophet, peace and blessings of Allah be upon him, said: "I am more entitled to Moses, and more entitled to the fasting of this day." Then he commanded his companions to fast.