in reading the auto-biography of the Kerala social reformer V.T. Bhattathiripad (1896-1982) born in a Nambudiri brahmin family (My Tears, my Dreams - Kanneerum Kinaavum, New Delhi: Oxford UP, 2013, translated from the Malayalam), sprinkled with Sanskrit verses (from the author's memory) in approximative transliteration (unfortunately I have not yet the original, with correct Sanskrit, version), I come across the following śloka (p. 27, corrected):

daivādhīnaṃ jagat sarvaṃ  mantrādhīnaṃ tu daivatam |

tan mantraṃ brāhmaṇādhīnaṃ  brāhmaṇo mama daivatam ||

with the author's comment "I don't know which idiot made up this verse in Sanskrit".

From a first search for, I found the same verse in Sāyaṇa's commentary on the Vaṃśabrāhmaṇa (ed. A.C. Burnell, Mangalore, 1873), p. 2 of the bhāṣya

with the variant in d "nama [sic] devatā" and the note by Burnell: "I cannot identify this piece of insolence. It is always in the mouths of S. Indian Brahmans."

The verse is also to be found in (the same) Sāyaṇa's Subhāṣitasudhānidhi (crit. ed. K. Krishnamooorthy, Dharwar, 1968, p. 22, 5.5,  with the variant in d "daivataṃ mahat": https://archive.org/details/21subhashitasudhanidhiofsayanacarya_201908/page/n25 ), which itself relies on the earlier (13th-14th c.?) South-Indian anthology Sūktiratnahāra (ed. K. Sambasiva Sastri, TSS 141, 1938, p. 6, 5.5, with the same variant in d "daivataṃ mahat": https://archive.org/details/in.ernet.dli.2015.284105/page/n21 - https://archive.org/details/in.ernet.dli.2015.493619/page/n27 )

Would somebody be aware of other, and especially earlier, occurrences in Sanskrit literature of this "traditional" verse (e.g. here quoted in a Tamil brahmins forum: https://www.tamilbrahmins.com/threads/temple-renovations.27311/  - note that the last pāda "brāhmaṇo mama daivatam" = also Viṣṇudharma 52.20 ed. Gruenendahl on GRETIL).