Of course the wisdom of that particular saying does not depend upon its source. And I do not think it is without wisdom. Many of us have been on the receiving end of words spoken in the name of the gospel by someone whose life or attitude did not "preach" the gospel. Our lives must bear witness to the good news of Jesus before our words about that good news can make any sense. But to suggest that the gospel can be preached without ever using words is deceptive. We ought to be able to tell the Story that makes the story of our lives make sense. That requires words as well as actions.
If we use this saying attributed to St. Francis as an excuse to never speak words of the gospel to others, it is rather like saying, as one wag has it, "Feed the hungry; if necessary use food."
And if we attribute only this saying to Francis, we will misrepresent the fact that he, himself, actually used words -- and used them boldly -- to preach the gospel.
Here is a story from the life of Francis of Assisi (emphasis mine):
The people of Gubbio, a town north of Assisi, were troubled by a huge wolf that attacked not only animals but people, so that the men had to arm themselves before going outside the town walls. They felt as if Gubbio were under siege.(as told by Jim Forest in The Ladder of the Beatitudes, p. 116-117)
Francis decided to help, though the local people, fearing for his life, tried to dissuade him. What chance could an unarmed man have against a wild animal with no conscience? But according to the Fioretti, the principal collection of stories of the saint’s life,
Francis placed his hope in the Lord Jesus Christ, master of all creatures. Protected neither by shield or helmet, only arming himself with the sign of the Cross, he bravely set out of the town with his companion, putting his faith in the Lord who makes those who believe in him walk without injury on an asp… and trample not merely on a wolf but even a lion and a dragon.Some local peasants followed the two brothers, keeping a safe distance. Finally the wolf saw Francis and came running, as if to attack him. The story continues:
The saint made the sign of the Cross, and the power of God… stopped the wolf, making it slow down and close its cruel mouth. Then Francis called to it, “Brother Wolf, in the name of Jesus Christ, I order you not to hurt me or anyone.”
The wolf then came close to Francis, lowered its head and then lay down at his feet as though it had become a lamb. Francis then censured the wolf for its former cruelties, especially for killing human beings made in the image of God, thus making a whole town into its deadly enemy.
“But, Brother Wolf, I want to make peace between you and them, so that they will not be harmed by you any more, and after they have forgiven you your past crimes, neither men nor dogs will pursue you anymore.”
The wolf responded with gestures of submission “showing that it willingly accepted what the saint had said and would observe it.”
Francis promised the wolf that the people of Gubbio would henceforth “give you food every day as long as you shall live, so that you will never again suffer hunger.” In return, the wolf had to give up attacking both animal and man. “And as Saint Francis held out his hand to receive the pledge, the wolf also raised its front paw and meekly and gently put it in Saint Francis’s hand as a sign that it had given its pledge.”
Francis led the wolf back into Gubbio, where the people of the town met them in the market square. Here Francis preached a sermon in which he said calamities were permitted by God because of our sins and that the fires of hell are far worse than the jaws of a wolf, which can only kill the body. He called on the people to do penance in order to be “free from the wolf in this world and from the devouring fire of hell in the next world.” He assured them that the wolf standing at his side would now live in peace with them, but that they were obliged to feed him every day. He pledged himself as “bondsman for Brother Wolf.”