Diogenes Laertius 6.37 (on Diogenes the Cynic, tr. R.D. Hicks):
One day, observing a child drinking out of his hands, he cast away the cup from his wallet with the words, "A child has beaten me in plainness of living."
Ausonius versified the episode in his Epigrams
Satchel, barley-meal, cloak, staff, cup -- this was the scanty kit of the Cynic. But he thought it was excessive. For seeing a rustic drinking with cupped hands, he said, "Why, cup, do I carry you, a superfluous thing?"
Pera, polenta, tribon, baculus, scyphus, arta supellex
ista fuit Cynici, set putat hanc nimiam.
Namque cavis manibus cernens potare bubulcum
'cur, scyphe, te', dixit, 'gesto supervacuum?'