Deuteronomy 16:10
And thou shalt keep the feast of weeks unto the Lord thy God with a tribute of a freewill offering of thine hand, which thou shalt give unto the Lord thy God, according as the Lord thy God hath blessed thee:
Cross-reference
Deuteronomy 16:16 includes the Feast of Weeks among the three required pilgrimages, adding the command to not come empty-handed.
Deuteronomy 16:17 repeats the same principle of giving in proportion to blessing, directly reinforcing the command for the Festival of Weeks.
Deuteronomy 16:9 gives the counting instruction that directly precedes the feast and offering command in verse 10 — a sequential link.
Exodus 34:22 also commands the Feast of Weeks, linking it to the wheat harvest — the same festival for which a freewill offering is prescribed here.
Leviticus 23:15 instructs counting seven weeks to determine the Feast of Weeks — the very festival for which this verse requires a freewill offering.
In Joel 1:16, joy and gladness are cut off from God's house — contrasting the commanded rejoicing at the Feast of Weeks here.
Judges 21:19 mentions a yearly feast at Shiloh — a historical example of the kind of festival observance commanded here for the Feast of Weeks.
1 Chronicles 29:9 shows people rejoicing over their freewill offerings for the temple — the same spirit of voluntary giving required here.
Proverbs 3:9 commands honoring God with firstfruits, similar to the freewill offering given according to blessing in the Feast of Weeks.
2 Chronicles 31:14 describes Levites overseeing freewill offerings — the same type of offering prescribed here for the Feast of Weeks.