Chullin 105a
חולין ק״ה א — ד"ה בְּעָא מִינֵּיהּ רַב אַסִּי מֵרַבִּי יוֹחָנָן
Chullin 105a:6
The Gemara discusses the separation between meat and milk and the requirement to wash the mouth and hands before eating dairy. Later authorities use this sugya as the starting point for the custom to wait after meat.
בְּעָא מִינֵּיהּ רַב אַסִּי מֵרַבִּי יוֹחָנָן: כַּמָּה יִשְׁהֶה בֵּין בָּשָׂר לִגְבִינָה? אָמַר לוֹ: וְלֹא כְּלוּם. אִינִי? וְהָא אָמַר רַב חִסְדָּא: אָכַל בָּשָׂר – אָסוּר לֶאֱכוֹל גְּבִינָה, גְּבִינָה – מוּתָּר לֶאֱכוֹל בָּשָׂר! אֶלָּא כַּמָּה יִשְׁהֶה בֵּין גְּבִינָה לְבָשָׂר? אֲמַר לֵיהּ: וְלֹא כְּלוּם. גּוּפָא, אָמַר רַב חִסְדָּא: אָכַל בָּשָׂר – אָסוּר לֶאֱכוֹל גְּבִינָה, גְּבִינָה – מוּתָּר לֶאֱכוֹל בָּשָׂר. אֲמַר לֵיהּ רַב אַחָא בַּר יוֹסֵף לְרַב חִסְדָּא: בָּשָׂר שֶׁבֵּין הַשִּׁינַּיִם מַהוּ?
§ Rav Asi posed a dilemma to Rabbi Yoḥanan: How much time should one wait between eating meat and eating cheese? Rabbi Yoḥanan said to him: No time at all. The Gemara asks: Is that so? But doesn’t Rav Ḥisda say: If one ate meat, it is prohibited for him to eat cheese immediately, but if he ate cheese it is permitted for him to eat meat without delay? Rather, Rav Asi actually asked Rabbi Yoḥanan the following question: How much time should one wait between eating cheese and eating meat? In response to this question, Rabbi Yoḥanan said to him: No time at all. After tangentially citing a statement of Rav Ḥisda, the Gemara discusses the matter itself. Rav Ḥisda says: If one ate meat, it is prohibited for him to eat cheese immediately, as the meat contains fatty substances that stick to one’s mouth and preserve the flavor of meat. But if he ate cheese it is permitted for him to eat meat without delay. Rav Aḥa bar Yosef said to Rav Ḥisda: In the case of meat that is between the teeth, what is the halakha? Are these remnants considered meat to the extent that one may not eat cheese as long as they are in his mouth?