This line is probably the best summary of Gandhi's philosophy of satyagraha as you can get in 16 words. But there's no evidence that the Great Soul ever said this.
We don't know where this quote came from, but it is strikingly similar to something that the trade unionist Nicholas Klein gave in a 1918 address to the Amalgamated Clothing Workers of America in Baltimore:
"First they ignore you. Then they ridicule you. And then they attack you and want to burn you. And then they build monuments to you. And that, is what is going to happen to the Amalgamated Clothing Workers of America."
[Editor's note: An earlier version gave the wrong year for Mr. Klein's speech.]