NarayanAt brahmA jAyate
NarayanAt rudro jAyate
This Upanishad was cited by Shankaracharya in his commentary on the Vishnu Sahasranama according to this site:
NArAyaNa Upanishad - Cited by Adi Shankara in Vishnu Sahasranama BhASya
Hence, it is an authentic Upanishad accepted by all ancient Vaidikas. The idea that it was an Upanishad composed later was started by Western Indologists:
It 'line of thinking' was started and propagated by biased and paid members of British East East Company like Max Muller, who in his translation and commentary on principle upanishads titled 'Th Upanishads - Vol 1 and 2' in intro page 83-84 attests views and opinions of another indologist where it is mentioned that Adi Sankara quoted for a late text / recent upanishad 'nArAyaNa upanishad'. Their challenging Adi Sankara is unacceptable. All consider him as an authority on spiritual matters.
My question is, where does Shankaracharya cite this Upanishad in his Vishnu Sahasranama Bhashya?