We but mirror the world. All the tendencies present in the outer world are to be found in the world of our body. If we could change ourselves, the tendencies in the world would also change.

From an article on dealing with snake bites, published in Indian Opinion dated August 9th, 1913, in the Gujarti language edition, English translation available in the Collected Works of Mahatma Gandhi, vol. 12, April 1913 - December 1914, p. 158.

A New York Times article as well as various other sources (e.g. Quote Investigator, Jennifer L. Phillips, Ph.D.) see this passage as similar to, "We need to be the change we wish to see in the world" even if Gandhi did not say those exact words.