Avodah Zarah 4:8
Avodah Zarah4: 8
לוֹקְחִין גַּת בְּעוּטָה מִן הָעוֹבֵד כּוֹכָבִים אַף עַל פִּי שֶׁהוּא נוֹטֵל בְּיָדוֹ וְנוֹתֵן לַתַּפּוּחַ, וְאֵינוֹ נַעֲשֶׂה יֵין נֶסֶךְ עַד שֶׁיֵּרֵד לַבּוֹר. יָרַד לַבּוֹר — מַה שֶּׁבַּבּוֹר אָסוּר, וְהַשְּׁאָר מֻתָּר.
One may buy a trodden winepress from an idolater, even though he takes with his hand and places in the mound. And it does not become nesech-wine until it descends into the cistern. [If] it descended to the cistern, that which is in the cistern is prohibited, and the remainder is permitted.