Esther 9:27-28

the Jews firmly obligated themselves and their offspring and all who joined them, that without fail they would keep these two days according to what was written and at the time appointed every year, that these days should be remembered and kept throughout every generation, in every clan, province, and city, and that these days of Purim should never fall into disuse among the Jews, nor should the commemoration of these days cease among their descendants.