Likutei Moharan 86:2/ליקוטי מוהר"ן פ״ו:ב׳

12. This is the explanation [of the opening verse]: {“Assist those who cease work on Shabbat…; who stride on it with short steps; so’adim (who feast) on it three times in order to bless. May their tzedek (righteousness) shine forth like the light of the seven days.”}

2who stride on it with short steps—That is, on Shabbat he still takes short steps, as explained above.

3So’ADim on it—That is, by means of Shabbat, which is the aspect of truth, he creates for himself SaAD (support) and a third leg.

4three times in order to bless—These are the three legs, as explained above. The way to strengthen them, so that he can walk properly, is this: May their tzedek shine forth like the light of the seven days—That is, he should give Shabbat tzedakah, this being the aspect of the sun as it is destined to be renewed “like the light of the seven days,” as explained above.