From the verse to the cup
זָכוֹר אֶת יוֹם הַשַּׁבָּת לְקַדְּשׁוֹ
The Gemara (Pesachim ק״ו ע״א (106a)) expounds: "remember it over wine" - remembering is done in speech, through Kiddush.
According to Rambam (Shabbat 29:1) the Torah obligation is speech, and wine is a rabbinic enactment. Night Kiddush is primary; daytime Kiddush ("Kiddusha Rabba") is an enactment of honor.