Source, in full
This quote has been ascribed to Zeno on Twitter since 2015. It similarly appears in the 2018 novel, Secrets Under the Sun, by Nadia Marks. It could be a paraphrase of this line from Plato: "it is the first and best of all victories for a man to conquer himself" (Laws, i. 626E, Burges trans.). The Buddhist text, the Dhammapada, contains the line: "Greater in battle than the man who would conquer a thousand-thousand men, is he who would conquer just one—himself." (8.103)