All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we advise you, dear brother, to refer to fatwa 81899.
As for the cause of the Prophet's delay in observing the fast on the Ninth of Muharram –in order to be different from the Jews –till the end of his life, we quote Ibn Hajar's words in his book “Al-Fath”. “The Prophet was used to conforming with the People of the Book in matters that he was not given a Divine order (to differ with them), especially if such matters contradicted Pagan's actions. But, when the Prophet Muhammad conquered Makkah and Islam spread, he wanted to differ from the People of the Book as proved in sound narrations. At first, he agreed with them and said: “We have more right in Moses than you”. Then, he wanted to be different from them.”
Allah knows best.