We must become the change we want to see.

Mohandas Gandhi (1869-1948) Indian lawyer, anti-colonial nationalist, political ethicist [Mahatma Gandhi]

(Attributed)

Variants:

• "Be the change you wish to see in the world."
• "You must be the change you wish to see in the world."

Almost always attributed to Gandhi, but not found in any of his published works. More discussion here.