"You shall not bow down to them, nor serve them" (Exodus 20:5). Why is this said? Because it says, "and went and served other gods" (Deuteronomy 17:3), to render one liable for the service on its own and for the bowing on its own. Or perhaps one is not liable until he both serves and bows down? Therefore it says "you shall not bow down," to render him liable for the service on its own and for the bowing on its own.
Bowing and Serving Idols Are Two Separate Offenses
Yalkut Shimoni on Torah 287:1
(שמות כ ה-ו) לֹא תִשְׁתַּחֲוֶה לָהֶם וְלֹא תָעָבְדֵם, לָמָּה נֶאֱמַר, לְפִי שֶׁהוּא אוֹמֵר (דברים יז, ג) "וַיֵּלֶךְ וְיַעֲבֹד אֱלֹהִים אֲחֵרִים" לְחַיֵּב עַל הָעֲבוֹדָה בִּפְנֵי עַצְמָהּ, וְעַל הִשְׁתַּחֲוָיָה בִּפְנֵי עַצְמָהּ, אוֹ לֹא יְהֵא חַיָּב עַד שֶׁיַּעֲבֹד וְיִשְׁתַּחֲוֶה, תַּלְמוּד לוֹמַר לֹא תִשְׁתַּחֲוֶה וְכוּ', לְחַיֵּב עַל הָעֲבוֹדָה בִּפְנֵי עַצְמָהּ, וְעַל הַהִשְׁתַּחֲוָויָה בִּפְנֵי עַצְמָהּ.