Genghis Khan

Genghis Khan (c. 1162–1227) united the steppe tribes and built, in one lifetime, the largest contiguous land empire in history. He left no writings of his own, which is the whole problem with quoting him: everything credited to him reaches us through people who wrote it down decades or centuries later, usually in another language and often with an agenda. This saying is one of the better-documented ones — and it is still second-hand.