Mishnayos Eruvin Perek 8 Mishnah 1

How does one participate in the joining of Shabbat boundaries? One who wishes to establish a joining of Shabbat boundaries for himself and others places a barrel of food in the location he designates as their place of residence, and says: This is for all the residents of my town, for anyone who wishes to go on Shabbat to a house of mourning or to a house of a wedding feast situated beyond the Shabbat limit.Anyone who accepted upon himself while it was still day, i.e., before the onset of Shabbat, that he will rely on the eiruv, is permitted to rely upon it; but if one did so only after nightfall, he is prohibited to rely upon it, as the principle is that one may not establish an eiruv after nightfall.

כֵּיצַד מִשְׁתַּתְּפִין בַּתְּחוּמִין. מֵנִיחַ אֶת הֶחָבִית וְאוֹמֵר, הֲרֵי זֶה לְכָל בְּנֵי עִירִי, לְכָל מִי שֶׁיֵּלֵךְ לְבֵית הָאֵבֶל אוֹ לְבֵית הַמִּשְׁתֶּה. וְכֹל שֶׁקִּבֵּל עָלָיו מִבְּעוֹד יוֹם, מֻתָּר. מִשֶּׁתֶּחְשַׁךְ, אָסוּר, שֶׁאֵין מְעָרְבִין מִשֶּׁתֶּחְשָׁךְ: