I Corinthians 15:2, 58
I Corinthians 15:2 NKJV
by which also you are saved, if you hold fast that word which I preached to you—unless you believed in vain.
I Corinthians 15:58 NKJV
Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your labor is not in vain in the Lord.