Righteousness stands for ever

You will all undoubtedly be familiar with the refrain,

כִּי לְעוֹלָם חַסְדּוֹ

 for his mercy endureth for ever or

 for forever is his loving-kindness

(42 times in KJV - 34 of these in the psalms)
But did you ever note the apparent uniqueness of,

צִדְקָתוֹ עֹמֶדֶת לָעַד

his righteousness endureth for ever or

his righteousness stands in perpetuity

I think I noted it before, but was trying to check it out with my limited Hebrew searching tools. This phrase occurs three times in the Scripture and all of them are in the pair of acrostics psalm 111 - 112 following the crux psalm 110.  Psalm 110 is a favorite of the NT authors. See my raw data on Psalms in the NT here.  Psalm 112:9 is referenced in 2 Corinthians 9:9.

So I am wondering what prompted the poet, having read Psalm 110 to write Psalm 111-112 - clearly these two are from the same hand.

(Interesting that endureth is a false concordant sound. In the Hebrew, the word does not exist in the mercy passages. But stands does exist in psalms 111-112.)

I suspect that the stimulus for the poet is in Torah but I have not identified the verses. Possibly Phineas with related concepts of the everlasting priesthood (Numbers 25:13) and imputed righteousness (Psalm 106:31), but I wouldn't particularly want to follow that line of reasoning. Possibly Genesis 18 where at least stand and righteous occur in the same chapter. There's no obvious single choice... maybe when I get to these psalms, something will have turned up.