The Day of ʿĀshūrāʾ
“…whoever wishes may fast it, and whoever wishes may leave it.”
Both Ṣaḥīḥ collections likewise relate from ʿAbd Allāh ibn ʿUmar (may Allah be pleased with them) that:
“The people of the Age of Ignorance used to fast the Day of ʿĀshūrāʾ, and the Messenger of Allah (Allah bless and grant him peace) together with the Muslims fasted it before Ramadan was made obligatory. When Ramadan was prescribed, the Messenger of Allah (Allah bless and grant him peace) said, ‘ʿĀshūrāʾ is one of the days of Allah; whoever wishes may fast it, and whoever wishes may leave it.’”
Because the original reason for fasting ʿĀshūrāʾ was not imitation of the People of the Book, the Prophet’s statement, “We have a stronger claim to Moses than you,” is simply an affirmation of the fast itself and a declaration to the Jews that the deed by which they seek to follow Moses is one we perform as well—and thus we are more entitled to Moses than you.
As for reconciling this with the report that “he liked to agree with the People of the Book in matters about which he had received no command,” several answers may be given.
First: Such conformity belonged to an earlier phase; thereafter Allah abrogated it, legislated that he differ from the People of the Book, and commanded him to do so. In the ḥadīth itself it is stated that “at first he let his hair hang down in the same manner as they did, then later he parted it.” Hence parting the hair became a distinctive sign of the Muslims, and one of the conditions imposed on the ahl al-dhimmah was that they were not to part their hair.
Likewise, Allah initially prescribed facing Jerusalem in prayer, in agreement with the People of the Book; then He abrogated that and commanded facing the Kaʿbah. He also informed that the Jews and other foolish people would say,
“ What has turned them from the qiblah they used to face? ” (al-Baqarah 2:142).