Esther 9:27
The Jews ordained, and took upon them, and upon their seed, and upon all such as joined themselves unto them, so as it should not fail, that they would keep these two days according to their writing, and according to their appointed time every year;
Cross-reference
Esther 9:31 confirms the Purim days for themselves and their seed – a direct parallel to the ordinance in 9:27.
Esther 8:17 describes people becoming Jews ('joined themselves') – the exact same phrase used in 9:27 for those included in Purim.
Deuteronomy 29:14 includes future generations in the covenant—mirroring Purim's obligation on descendants and proselytes.
Deuteronomy 29:15 extends the covenant to those not present—similar to Esther binding future generations to keep Purim.
1 Samuel 30:25 describes David making a lasting statute—just as the Jews made Purim a permanent ordinance.
Zechariah 8:23 depicts Gentiles taking hold of a Jew to go with them, directly mirroring the 'joined themselves unto them' in Esther 9:27.
Isaiah 56:3 uses 'joined himself to the LORD' for foreigners, echoing the inclusion of 'such as joined themselves' in Esther 9:27.
Isaiah 56:6 again speaks of foreigners joining the LORD – a thematic parallel to proselytes joining the Jews for Purim.
Zechariah 2:11 prophesies nations joining the LORD – reminiscent of Gentiles joining the Jewish community in Esther 9:27.