This early Hasidic work frames the spiritual potency of the tzaddik and the importance of connecting to him in avodat Hashem. It is frequently used in discussions of visiting the resting place of a tzaddik to arouse devotion.
צוואת הריב"ש ע"ה. להיות תמים בעבודתו יתברך עבודה תמה ועיקר שלא לשכוח הדברים. ועיקר ללמוד בכל יום שיעור מוסר הן רב הן מעט. ולראות א"ע תמיד להדבק א"ע במדות טובות והנהגות ישרות ושלא יניח שום יום מעשיית מצוה הן קלה הן חמורה. וסימנך הוי זהיר במצוה קלה כבחמורה פירש זהיר מלשון והמשכילים יזהירו. ר"ל הנשמה תזהיר ותאיר ממצוה קלה כבחמורה כי רחמנא ליבא בעי:
Rabbi Yisrael Baal Shem, peace be upon him, taught us to be wholehearted in our service of HaShem, blessed is He and to serve Him with simplicity. Of primary importance is not to forget the Commandments we received directly from HaShem at Chorev (Aseret HaDvarim-עשרת הדברים), and to study teachings of Mussar, that deal with developing good character traits and right conduct, on a daily basis, whether one studies a little or a lot. Make sure to always adhere to good character traits and right conduct and do not let a day go by without doing a mitzvah-commandment, be it a minor mitzvah or a major one. The mnemonic by which to remember this is the verse, “Be as radiant-Zahir-זהיר from a minor mitzvah as from a major one.” The word Zahir-זהיר, which is usually translated as “careful,” is related, in its root, to the terminology of the verse, “And they that are wise will be radiant-Yazhiru-יזהירו as the radiance-Zohar-זהר of the firmament.” This is to say that the soul should be as radiant and illuminated by the performance of a minor mitzvah as from a major one, for “the Merciful One desires the heart.”