Isaiah 56:6
Also the sons of the stranger, that join themselves to the Lord, to serve him, and to love the name of the Lord, to be his servants, every one that keepeth the sabbath from polluting it, and taketh hold of my covenant;
Cross-reference
In Isaiah 56:2, the same blessing is tied to keeping the sabbath and holding fast — directly reinforcing the conditions here.
In Isaiah 56:3, the foreigner who joins the Lord is reassured he won't be separated — this verse expands on the same group's requirements.
In Isaiah 58:13, honoring the sabbath is further described as a delight — expanding on the sabbath-keeping required here.
Isaiah 14:1 prophesies foreigners joining Israel—a direct parallel to the inclusion of foreigners in Isaiah 56:6.
In Isaiah 44:5, people claim the Lord's name and identify with Israel — similar to foreigners here joining and serving the Lord.
In 1 Thessalonians 1:9, converts turned from idols to serve the living God — parallel to foreigners here serving the Lord and keeping his covenant.
In Jeremiah 50:5, exiles seek to join themselves to the Lord in an everlasting covenant — the same language of 'joining' used for foreigners here.
Jeremiah 3:17 prophesies all nations gathering to Jerusalem—a parallel to the inclusion of foreigners in Isaiah 56:6.
Romans 2:26 says uncircumcised who keep the law are regarded as circumcised—directly parallels the faithful foreigners in Isaiah who keep Sabbath and covenant.
Ezekiel 44:7 condemns uncircumcised foreigners profaning the sanctuary—opposite of the covenant-keeping foreigners accepted in Isaiah.
Ezekiel 20:13 describes Israel profaning Sabbaths in the wilderness—contrasting with the faithful foreigners here who keep the Sabbath.
Jeremiah 12:16 describes foreigners learning God's ways and being built up among His people—parallel to the covenant joining in Isaiah.
In Exodus 31:14, the sabbath is commanded as holy under penalty of death — the same sabbath law that foreigners here are required to keep.
Deuteronomy 5:12 is the Sabbath commandment from the Decalogue—the very law these foreigners are keeping.
Leviticus 23:3 gives the Sabbath command that these foreigners are said to keep—a central marker of covenant loyalty.
Ezekiel 47:22 grants sojourners inheritance among Israel—parallel to Isaiah's inclusion of foreigners who join the Lord and keep his covenant.
Esther 9:27 describes non-Jews joining the Jewish community to observe Purim—similar inclusion of outsiders as in Isaiah.
Zechariah 6:15 promises that 'far off' ones will help build the temple if obedient—similar to the foreign joiners in Isaiah who keep covenant.