The Hebrew Is Sufficient
Citing Scripture was a lot less formal in ancient times than it is in scholarly circles in modern times. People knew most of the Scriptures by heart, and in teaching and letters often incorporated themes and phrases from Scripture into a kind of tapestry at once their own argument, and at the same time interwoven and undergirded with Scriptural authority, force, and witness. I think the author is paraphrasing from a portion of 'The Prophets,' and not one particular book.
Here's a nifty little chart showing the proximity of the two verses:
1 2 3 1 2 3
Or in the 'grand scheme' of the Old Testament:
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Namely, I think he's quoting from:
הנה עפלה לא־ישרה נפשו בו וצדיק באמונתו יחיה׃
Behold the puffed up man: his soul is not right within him; but the just man lives by his faith.
And 42 verses later (ignoring the divisions of the books as was often done, treating them as one book):
ונטיתי ידי על־יהודה ועל כל־יושבי ירושלם והכרתי מן־המקום הזה את־שאר הבעל את־שם הכמרים עם־הכהנים׃ ... ואת־הנסוגים מאחרי יהוה ואשר לא־בקשו את־יהוה ולא דרשהו׃ הס מפני אדני יהוה כי קרוב יום יהוה כי־הכין יהוה זבח הקדיש קראיו׃
And I will stretch forth my hand upon Judah, and upon all that dwell in Jerusalem, and I will cut off the remnant of Baal therefrom: the idol-serving priest together with the Kohen [legitimate priests], ... and those who turn aside from the way of the Lord, and who seek him not, nor entreat him.
"those who shrink back" is a perfect translation of הנסוגים (those who turn away [i.e. from the Lord to other things], apostates). It has the sense, one who recoils also. Hence the corresponding Greek υποστελλω (to recoil; to shrink back/from).
Then, 'my soul shall have no pleasure in him' (cf. Jer 48:38 :"my soul" being a Semtism meaning "I") might be a paraphrase of the overall discontentment and mind to punish the people of God in Zephaniah, which possibly made it into the Septuagint by way of Christian interpolation which misunderstood this as a contiguous quote from Habakkuk. And the first person aspect comes from the fact that God is speaking in the first place, "I will cut off..." (v. ).
Yielding the following punctuation, if this is the correct evaluation, for Hebrews:
Hebrews 10:38 And, my 'Righteous one will live by faith,' but if he 'Shrinks back' my soul shall have no pleasure in him.