The rule and its reason
The Gemara (Shabbat ק״נ ע״א (150a)) states: "instructing a non-Jew is shevut". Rashi grounds it in "from pursuing your affairs and speaking words" - even speech about work harms Shabbat. Rambam (Shabbat 6:1) adds: so that Shabbat not become light in one's eyes and he come to do it himself.
אִם תָּשִׁיב מִשַּׁבָּת רַגְלֶךָ עֲשׂוֹת חֲפָצֶיךָ בְּיוֹם קָדְשִׁי
From this verse and its continuation Chazal learn about speech and business on Shabbat.