Here’s a good article that answers the question. (H/T Benjamin from SA)

Excerpt:

Francis of Assisi is said to have said, “Preach the gospel at all times; when necessary, use words.”

This saying is carted out whenever someone wants to suggest that Christians talk about the gospel too much, and live the gospel too little. Fair enough—that can be a problem. Much of the rhetorical power of the quotation comes from the assumption that Francis not only said it but lived it.

The problem is that he did not say it. Nor did he live it. And those two contra-facts tell us something about the spirit of our age.

Let’s commit a little history (let me un-humbly draw on some chapters from my biography of St. Francis).

First, no biography written within the first 200 years of his death contains the saying. It’s not likely that a pithy quote like this would have been missed by his earliest disciples.

Second, in his day, Francis was known as much for his preaching as for his lifestyle.

He began preaching early in his ministry, first in the Assisi church of Saint George, in which he had gone to school as a child, and later in the cathedral of Saint Rufinus. He usually preached on Sundays, spending Saturday evenings devoted to prayer and meditation reflecting on what he would say to the people the next day.

[…]Another early biography talked about how his preaching was received: “His words were neither hollow nor ridiculous, but filled with the power of the Holy Spirit, penetrating the marrow of the heart, so that listeners were turned to great amazement.”

As a result, he quickly gained followers, and it wasn’t long before he told his most devoted adherents to preach as well.

The article goes on to explain where this wimpy trend of not preaching using words comes from, and whether it matches the example of Jesus in the New Testament writings.