On Rosh Chodesh, after Hallel, there is the following:
:זְבַדְיָה יִשְׁמְרֵנִי וִיחַיֵּנִי, כֵּן רָצוֹן מִלְּֿפָנֶיךָ, אֱלֹהִים חַיִּים וּמֶלֶךְ עוֹלָם אֲשֶׁר בְּיָדוֹ נֶפֶשׁ כָּל חָי, אָמֵן
May Zevadyah protect me and grant me life. So may it be Your will, living God and eternal King, in whose hand is the soul of every living being, Amen.1
Who is Zevadyah? Why are we asking him to protect us and grant us life? What is the source for this passage?
Interestingly enough it is not Chabad custom to recite this verse, despite its inclusion in the siddur.
1 Siddur Tehillat Hashem, Annotated English Edition, p. 312