אורח חיים רנ״וOrach Chayim 256

Se’if 1א׳

כְּשֶׁהָיוּ יִשְׂרָאֵל בְּיִשּׁוּבָן, הָיוּ תּוֹקְעִין בְּעֶרֶב שַׁבָּת שִׁשָּׁה תְּקִיעוֹת, כְּדֵי לְהַבְדִּיל אֶת הָעָם מִן הַמְּלָאכָה. הַגָּה: וְנָהֲגוּ בַּקְּהִלּוֹת הַקְּדוֹשׁוֹת, שֶׁכָּל שֶׁהוּא סָמוּךְ לְשַׁבָּת כְּחֵצִי אוֹ שָׁעָה, שֶׁמַּכְרִיז שְׁלִיחַ צִבּוּר לְהָכִין עַצְמָן לְשַׁבָּת, וְהוּא בִּמְקוֹם הַתְּקִיעוֹת בִּימֵיהֶם, וְכֵן רָאוּי לִנְהֹג בְּכָל מָקוֹם.

The Six Shofar Blasts that Would Be Sounded on Friday: When the Jewish people were settled in their land, they used to sound six shofar blasts on Erev Shabbat, [to alert] the people to conclude [their] work. RAMA: At present, the custom in holy communities is that a half hour or an hour before Shabbos, an emissary of the community announces [that everyone] should prepare himself for Shabbat, and this is in place of the shofar blasts that took place in those days. It is appropriate to act thus in all places.