It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) came to Madinah Munawwarah and saw that the Jews were fasting on the day of Ashura. So he (peace and blessings be upon him) asked: "For what reason do you fast on this day?" They said: "This is a great day; on this day Allah Almighty saved the Children of Israel from their enemies, so Musa (peace be upon him) fasted on this day." The Prophet (peace and blessings be upon him) said: "I am more entitled to Musa (peace be upon him) than you." So he (peace and blessings be upon him) himself fasted on this day and ordered the people to fast on this day as well.

"I am more entitled to Musa (peace be upon him) than you." That is, I have more right to follow Musa (peace be upon him), because Allah Almighty commanded him (peace and blessings be upon him): {فَبِھُدَاھُمُ اقْتَدِہْ} ... So follow their guidance. From this, it is clear that the objective of the Prophet (peace and blessings be upon him) was to follow Musa (peace be upon him), not the Jews.

This removes the objection that the Prophet (peace and blessings be upon him) preferred to oppose the Jews rather than agree with them. In the beginning, the Prophet (peace and blessings be upon him) preferred to agree with them for the sake of winning their hearts, but when he (peace and blessings be upon him) saw that these People of the Book were persistent in disbelief and were not being affected by conciliation, he (peace and blessings be upon him) abandoned agreeing with them and inclined towards opposing them. That is why, in the end, the Prophet (peace and blessings be upon him) resolved to fast on the ninth of Muharram.