You are not capable of finding the source so here you go:
Lucian, The Ignorant Book Collector 4 (tr. H.W. and F.G. Fowler):
πίθηκος γὰρ ὁ πίθηκος, ἡ παροιμία φησί, κἂν χρύσεα ἔχῃ σύμβολα.
An ape is still an ape, says the proverb, though his trappings be of gold.
Claudian, Against Eutropius 1.303-306 (tr. Maurice Platnauer):
humani qualis simulator simius oris,
quem puer adridens pretioso stamine Serum
velavit nudasque nates ac terga reliquit,
ludibrium mensis.
'Twas as though an ape, man's imitator, had been decked out in sport with precious silken garments by a boy who had left his back and quarters uncovered to amuse the guests at supper.
Spanish Proverb:
Aunque la mona se vista de seda, mona se queda.
Although the monkey dresses in silk, she remains a monkey.
Erasmus, Praise of Folly 17:
iuxta Graecorum proverbium, simia semper est simia, etiam si purpura vestiatur.
According to the proverb of the Greeks, an ape is always an ape, even if it's dressed in purple.