Deuteronomy 23:3

Overview - Deuteronomy 23
Who may or may not enter into the congregation.
Uncleanness is to be avoided in the host.
15 Of the fugitive servant.
17 Of filthiness.
18 Of abominable sacrifices.
19 Of usury.
21 Of vows.
24 Of trespasses.
Treasury of Scripture Knowledge

Deuteronomy 23:3  (King James Version)
An Ammonite or Moabite shall not enter into the congregation of the LORD; even to their tenth generation shall they not enter into the congregation of the LORD for ever:

These nations were subjected for their impiety, wickedness, and enmity to Israel, (ver. 4, 5,) to peculiar disgrace; and on this account were not permitted to hold any office among the Israelites. This did not, however, disqualify them from becoming proselytes; for Ruth, who was a Moabitess, was married to Boaz, and became one of the progenitors of our Lord.
Ruth 4:6 Ruth 4:10-22 ; Nehemiah 4:3 Nehemiah 4:7 ; Nehemiah 13:1 Nehemiah 13:2 Nehemiah 13:23 Isaiah 56:3