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 very good 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 to his Companions (may Allah be pleased with them): "You have more right to Musa (peace be upon him) than they do, so you should also fast on this day."