One who trusts in H'shem before choosing an occupation makes sure it will allow him to practice his religious and secular beliefs and obligations always focused on his ultimate reward. His intention in earning a living is never to feed his material desires, yet to fulfill his obligations to H'shem and his religious duties. He knows that when he takes these precautions his livelihood will lead him to live a life of health and not of sickness
pg451
Nefesh Hachaim
Just imagine what would happen to an observing Jew that would fall prey to the persuasions of the Yetzer Hara. Imagine the Yetzer would convince him, his kavana, and intentions were more important than fulfilling the mitzvah at the obligated time. What would come of trying to blow the shofar, fast on Yom Kippur, shake lulav and esrog, build a succa or eat his matzah? All these mitzvot are time-restricted. The Yeter Hara wants to make the Tzadik miss all of these obligations by making him focus his intentions on the wrong aspect of doing the mitzvah.
Prologue to Gate Four, Chapter 4 pg 126