Many sayings are attributed to St. Francis, but were never actually written by him. One of the more famous of these has to do with preaching without words. This one is based on what St. Francis said and taught.
What St. Francis said about giving good example, and how this was more important that preaching with words comes from a variety of sources, on of which is:
The Little Flowers of St. Francis: Chapter 50
At the end of this chapter, in the English translation by T. Okey, which appeared in the Everyman Library edition of 1951, Philosophy and Theology, n. 485A, p. 295, St. Francis is quoted to have said:But as for me, I desire this privilege from the Lord, that never may I have any privilege from man, except to do reverence to all, and to convert the world by obedience to the Holy Rule rather by example than by word.
"The Holy Rule" is the Rule of St. Francis.