St. Francis of Assisi is often accredited as saying, “Preach the gospel at all times. Use words if necessary.” But nowhere can this catchy phrase be found in his writings.
Father Pat from American Catholic has this to say.
This is a great quote, very Franciscan in its spirit, but not literally from St. Francis. The thought is his; this catchy phrasing is not in his writings or in the earliest biographies about him. In Chapter XVII of his Rule of 1221, Francis told the friars not to preach unless they had received the proper permission to do so. Then he added, “Let all the brothers, however, preach by their deeds.” 
After being a Franciscan for 28 years and earning an M.A. in Franciscan studies he eventually heard the “Use words if necessary” quote in 1996. He continues,
About a year ago, a friend of mine used the Internet to contact some of the most eminent Franciscan scholars in the world, seeking the source of this “Use words if necessary” quote. It is clearly not in any of Francis’ writings. After a couple weeks of searching, no scholar could find this quote in a story written within 200 years of Francis’ death.
Why is this accredited to St. Francis? Partially because it so thoroughly reflects his spirit, but mostly because had Joe Blogs said it, it wouldn’t have become so widespread. A parallel example is the “Peace Prayer”, where the oldest known copy dates to 1912 in France, but is nevertheless attributed to St. Francis to guarantee a wide diffusion of the text.
Ray Comfort complains this phrase has been used to justify not speaking out the truth content of the gospel. He says,
I regularly meet those who think they can obey the Great Commission without using words. . . . With a little probing, they are the relationship folks, who think preaching the Gospel means building relationships with the lost, and never mentioning words like “sin,” “Hell,” and “Judgment Day.” 
He comments that real love is not withholding the Bread of Life to those who are starving to death. Francis was a loving man who was not afraid to use words when he preached. He wasn’t frightened to preach repentance to a sinful world. I conclude therefore, that he most likely understood the following Bible verses to mean the use of words in the Gospel proclamation is necessary.
“How then shall they call on Him in whom they have not believed? And how shall they believe in Him of whom they have not heard? And how shall they hear without a preacher?” (Romans 10:14)
“Wherefore I take you to record this day, that I am pure from the blood of all men. For I have not shunned to declare unto you all the counsel of God” (Acts 20: 26-27; cf. Ezekiel 3:18)