60% sureLots of sources credit Aesop with this, and they agree with each other. The book or the speech it first came from hasn't turned up yet.
What turned up
The Tortoise and the Hare (Aesop, Perry Index 226) is genuine, but the pithy English phrase is a later paraphrase — the original Greek moral reads differently.
gutenberg.orgChecked using claude-fable-5, 22 August 2026Three Hundred Aesop's Fables, trans. George Fyler Townsend (public domain; full text grepped 2026-08-22). Printed moral of The Hare and the Tortoise: 'Slow but steady wins the race.' — stored text is the popular variant ('and' for 'but'). The moral line is Townsend's English, not Aesop's Greek.