What is the origin (i.e earliest usages) of appending הי"ד ("May God Avenge His Blood") after the name of a murder victim.
I heard from someone a while ago (wish I could recall whom; might have been something I read in Art Scroll siddur or Birnbaum?) it is from:
Deuteronomy 32:43 (Sefaria English):
הַרְנִ֤ינוּ גוֹיִם֙ עַמּ֔וֹ כִּ֥י דַם־עֲבָדָ֖יו יִקּ֑וֹם וְנָקָם֙ יָשִׁ֣יב לְצָרָ֔יו וְכִפֶּ֥ר אַדְמָת֖וֹ עַמּֽוֹ׃ (פ)
Sing aloud, O ye nations, of His people; For He doth avenge the blood of His servants, And doth render vengeance to His adversaries, And doth make expiation for the land of His people.
Rashbam on Deuteronomy 32:43:2:
וכפר אדמתו עמו - יקנח דם עמו מן האדמה על ידי שפיכת דם אויבים. כדכתיב: ולארץ לא יכופר לדם אשר שפך בה כי אם בדם שופכו.
וכפר אדמתו עמו, He will cleanse the blood of His people which has been spilled and been absorbed by the earth, by spilling the blood of their enemies, as the Torah’s recipe for cleansing the holy soil of the Land of Israel from innocently spilled blood (Numbers 35,33).
Even though, there is nothing explicitly relating the Torah verse to the expression, there are strong similarities to the wording in the two. To me, it seems credible that this verse is probably the source of the expression.