The Upanishads, Part 2 (SBE15), by Max Müller, , at sacred-texts.com
THIS Upanishad is called the Prasña or Shat-prasña-upanishad, and at the end of a chapter we find occasionally iti prasñaprativakanam, i.e. thus ends the answer to the question. It is ascribed to the Atharva-veda, and occasionally to the Pippalâda-sâkhâ, one of the most important sâkhâs of that Veda. Pippalâda is mentioned in the Upanishad as the name of the principal teacher.
Saṅkara, in the beginning of his commentary, says:
[paragraph continues] Mantroktasyârthasya vistarânuvâdidam Brâhmanam ârabhyate, which would mean 'this Brâhmana is commenced as more fully repeating what has been declared in the Mantra.' This, however, does not, I believe, refer to a Mantra or hymn in the Atharva-veda-samhitâ, but to the Mundaka-upanishad, which, as written in verse, is sometimes spoken of as a Mantra, or Mantropanishad. This is also the opinion of Ânandagiri, who says, I one might think that it was mere repetition (punarukti), if the essence of the Self, which has been explained by the Mantras, were to be taught here again by the Brâhmana.' For he adds, 'by the Mantras "Brahma devânâm," &c.,' and this is evidently meant for the beginning of the Mundaka-upanishad, 'Brahmâ devânâm.' Ânandagiri refers again to the Mundaka in order to show that the Prasña is not a mere repetition, and if Saṅkara calls the beginning of it a Brâhmana, this must be taken in the more general sense of 'what is not Mantra 1.' Mantropanishad is a name used of several Upanishads which are written in verse, and some of which, like the Îsâ, have kept their place in the Samhitâs.
xliii:1 Mantravyatiriktabhâge tu brâhmanasabdah, Rig-veda, Sâyana's Introduction, vol. i, p. 23.