[Advaita-l] The source of the word Dvija in Vedas

[K Ram]

Pranams to all the experts in the group. This maybe slightly off topic from
a strict Advaitic or Vedantic standpoint, but I'm trying to find the source
of the word "Dvija" being mentioned in Vedas either in Karma Kanda or
Upasana Kanda.

Since Dvija sets the tone for all of Purva Mimamsa practices and rituals,
I'm trying find the Shastra Pramanam for it in Vedas. To be clear, I've
seen it being mentioned in Manu Smriti but not in Shruthi (including the
Upanishads) and hence the need for clarity.

Can someone please throw some light on this?

Namaskaram
-Karthik Ram