Ephesians 2:8-9: "8 For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, 9 not a result of works, so that no one may boast."
Romans 3:28: "For we hold that one is justified by faith apart from works of the law."
Romans 4:5: "And to the one who does not work but believes in him who justified the ungodly, his faith is counted as righteousness"
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The text is public domain:
Not what these hands have doneCan save this guilty soul;Not what this toiling flesh has borneCan make my spirit whole.
Not what I feel or doCan give me peace with God;Not all my prayers and sighs and tearsCan bear my awful load.
Thy work alone, O Christ,Can ease this weight of sin;Thy blood alone, O Lamb of God,Can give me peace within.
Thy love to me, O God,Not mine, O Lord, to Thee,Can rid me of this dark unrestAnd set my spirit free.
Thy grace alone, O God,To me can pardon speak;Thy pow'r alone, O Son of God,Can this sore bondage break.
I bless the Christ of God,I rest on love divine,And with unfalt'ring lip and heartI call this Savior mine.
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The hymn text seems to draw from Ephesians 2:8-9 a bit more than from the other two passages, but all three are fairly similar.
The name "Lamb of God" in the third verse comes from John 1:29.