סימן עז סעיף ב
יְכוֹלִין לְקַדֵּשׁ וְלֶאֱכוֹל אַף עַל פִּי שֶׁעֲדַיִן אֵינוֹ לַיְלָה. אַךְ הַנּוֹהֲגִין כָּל יְמוֹת הַחֹל לְהִתְפַּלֵּל מַעֲרִיב בִּזְמַנָּהּ וּבְשַׁבָּת מַקְדִּימִין (כְּמוֹ שֶׁכָּתַבְתִּי בְּסִימָן שֶׁלִפְנֵי זֶה), אֵלּוּ אֲסוּרִין לֶאֱכוֹל מִשֶּׁהִגִּיעַ חֲצִי שָׁעָה קֹדֶם צֵאת הַכּוֹכָבִים. וְלָכֵן אִם אֵין יוֹתֵר מֵחֲצִי שָׁעָה עַד הַלַּיְלָה, צְרִיכִין לְהַמְתִּין עַד הַלַּיְלָה, וְאָז יִקְרְאוּ תְּחִלָּה שָׁלֹשׁ פָּרָשִׁיּוֹת שֶׁל קְרִיאַת שְׁמַע, וְאַחַר כָּךְ יְקַדֵּשׁ. וְאָסוּר לִטְעֹם כְּלוּם וַאֲפִלּוּ מַיִם קֹדֶם קִדּוּשׁ.
You may recite kiddush and eat the Shabbos meal even though it is not yet night.4Mishnah Berurah 267: 1 says it is preferable to eat at least one kazayis of bread after nightfall, which is also the ruling of Sefer Chassidim and Maharal. However, those accustomed to pray Maariv at the proper time, during weekdays but who pray earlier on Shabbos, are forbidden to eat after one half-hour before the stars appear.5See Magein Avraham 267: 2. However one should not rebuke those who are lenient. Also see Sha’ar Hatziyun 267: 11. If therefore it is only one-half hour until nightfall, they must wait until nightfall. And then they should first read the three chapters of Shema and after that recite kiddush. It is forbidden to eat anything, even (to drink) water before kiddush.6This is true once it gets dark or if you ushered in the Shabbos while it was yet daytime [Magein Avraham 271: 5]. At twilight it is also forbidden, see Magein Avraham 271: 10. But Daas Torah rules that one may be lenient in case of discomfort from thirst.7You may rinse your mouth with more than a revi’is of water but with other liquids only less than a revi’is. [Peri Megadim 271: 5].
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)