He was as careful in his speech as in his actions. It goes without saying that he refrained from whatever was prohibited by the halachah.
For this we have Rabbi Israel [Salanter's] own testimony. Upon reproving one of his disciples for his words, Rabbi Israel [Salanter] remarked: "Insofar as evil gossip (lashon hara) is concerned, you cannot tell me, 'Remove a beam from between your eyes,' and, it seems, not with respect to idle chatter either."
But even in speech that is permitted, he avoided superfluous verbiage and would weigh and count his words to make them conform to standards of propriety and refinement. One of the scholars of the generation observed on a specific occasion: "Rabbi Israel [Salanter] does not squander words. Every sound or word that issues from his mouth is first considered and reflected on. He purifies them like a silver smelter and weighs them in a chemical balance." -- The Mussar Movement Volume 1, Part 2 page 197
Not only was Rabbi Israel [Salanter] opposed to the performance of the finer points of mitzvot at the expense of human beings, he held that one had no right even to perform the essentials of a mitzvah or even extricate himself from grievous sin if he thereby inflicted suffering on someone else.
A question was submitted to him: Someone had sinned in secret against a friend of his by speaking evil of him. Was it permissible for this person now to go to his friend and seek forgiveness? In so doing, however, he would have to disclose what he had said to the friend he had maligned.
Rabbi Israel [Salanter] ruled, that although the questioner would absolve himself from grievous guilt by seeking his friends forgiveness, he had no right to pursue his own good by hurting his friend - enhancing his righteousness at the expense of causing distress to someone else. This is the extent to which R. Israel took the feelings of others into account, how he engaged in complicated calculations so as to avoid giving any hurt or distress to others. -- The Mussar Movement, Volume 1, Part 2 pages 229 - 230
A famous Chasidic story illustrates the extent of the damage that Lashon HaRa can do: A man went about gossiping and telling malicious stories without restraint. Later, when he realized how much his tales had hurt people, and began to feel remorse. He went to the Rabbi seeking repentance, saying he would do anything he could to make amends. The Rabbi told the man, “Take a feather pillow, cut it open, and scatter the feathers into the wind.” The man thought this was a bizarre request, but it was simple enough, and so he did it. When he returned to inform the rabbi that the task was done, the Rabbi said, “Now, go and collect all the feathers and return them to the pillow.” Again, the man went to do as the rabbi had asked, but found that the feathers had blown far and wide, and he was unable to retrieve even a handful. He returned to the rabbi, ashamed to admit he was not able to gather the feathers, certain that he should never have released the feathers in the first place. Knowingly, the rabbi rebuked him, saying, “Your words are like the feathers: once they leave your mouth, you know not where they will go, and you can never retrieve them back again. It is always wiser to guard your tongue and keep your words to yourself.”