It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) arrived in Madinah, he saw the Jews observing the fast on the tenth of Muharram. The Prophet (peace and blessings be upon him) asked them, "What is the special reason for which you fast on this day?" They replied, "This is a great day; on this day Allah granted deliverance to the Children of Israel from their enemy, so Musa (peace be upon him) fasted on this day." The Prophet (peace and blessings be upon him) said, "I have more right to Musa than you." So the Prophet (peace and blessings be upon him) himself fasted on this day and also ordered his Companions (may Allah be pleased with them) to fast on this day.