Dr. Alan Morinis sites a passage in the name of RYS both in the beginning (page 18) and the end (page 222) of Climbing Jacobs Ladder.

“As long as one lives a life of calmness and tranquility in the service of God, it is clear that he is remote from true service.”

The line is actually from an article written by Prof. Immanuel Etkes, titled, “Rabbi Yisrael Salanter and His Pyschology of Mussar” that was included in a book titled Jewish Spirituality Vol 2.
The quote from there (thanks to Google Books) is:
“But as long as one lives a life of calmness and tranquility in the service of God it is clear that he is remote from true service…For it is impossible that a person not encounter things which he desires and which the Torah prohibits.” (Kokhvey Or, 187)

In Prof. Etkes’ book, Rabbi Israel Salanter and the Mussar Movement, he cites a more extensive version of the more the quote from Kokhvey Or (187) in the name of R Simcha Zissel Ziv:
“It is certainly understood that a man is not considered as a human being if he lives a life of ease and does not labor in fear, for this is the whole duty of man…But so long as a person is living a life of tranquility in the service of Creator, he ought to know that he has surely shaken off and is empty of the service of God, may He be blessed…for it is impossible that person not daily encounter circumstances in which his desire and [the command] the Torah conflict with one another…”