1 Corinthians 1:29 gives the reason: to eliminate human boasting, so boasting must be only in the Lord.

Jeremiah 9:23 warns against boasting in wisdom, might, or riches—the negative foil to Paul's positive command, setting up the quote in verse 24.

Jeremiah 9:24 is the direct source Paul quotes: 'let him who boasts boast in this, that he understands and knows me.'

In 2 Corinthians 10:17, Paul cites the same 'boast in the Lord' maxim again, reinforcing the principle in a different context.

Galatians 6:14 exemplifies boasting in the cross of Christ—a direct application of the 'boast in the Lord' principle Paul quotes.

Philippians 3:3 says believers 'glory in Christ Jesus'—a clear parallel to boasting in the Lord, with no confidence in the flesh.

Psalm 34:2 explicitly says 'My soul makes its boast in the Lord' — the very phrase Paul summarizes. Strong thematic and verbal parallel.

1 Chronicles 16:10 calls to 'glory in his holy name' — the same idea of boasting in the Lord that Paul cites.

Isaiah 45:25 declares that in the Lord Israel shall be justified and glory—a parallel call to boast only in God, though not a direct quote.

Psalm 105:3 says 'glory in his holy name' — identical to 1 Chronicles 16:10 and parallel to boasting in the Lord.