Who's "Zevad-yah?"

Following the completion of hallel on Rosh Chodesh, many have the custom to recite the passuk "ואברהם זקן בא בימים..." followed by the mention of "Zevad-yah," that He should guard us and give us life, etc.
Where does this come from and who is "Zevad-yah?"
R. Avraham Dovid Lavat in his book "Sha'ar Hakollel" (סי' ל"ז סק"ט-י) says that this is not found in any of the works of the Ariza"l, and for this reason, the Alter Rebbe in Siddur writes that "there are those accustomed to saying the following."
Where does it indeed originate?
R. Emanuel Chai Riki, a late seventeenth century Kabbalist, authored a sefer on kabbalah entitled "Mishnas Chassidim." There he writes that if one wishes long life, they should say the above mentioned passuk after hallel. He continues to explain that the first letter of each of the words of this passuk spell out a unique name of Hashem - "זבדי-ה": The ז of זקן, the ב of בא, the מ of the word בימים is switched with the letter ד (according to the cheshbon of איק, בכר), and the final two letters "י-ה" from Hashem's name mentioned next in the passuk.
He concludes that the name should be pronounced "Zevud-yah" - with a קמץ under the ב. The spelling with a פתח appears as a man's name in עזרא ח, ח.
משנת חסידים מסכת ראש חודש פ"ב מ"ו-ז
ויש אומרים שהוא שם מלאך, עי' שער הכולל שם ובספר כף החיים סי' ה' סק"י בשם האריז"ל
See here