Zevachim 21B

איכא בינייהו גזירת שיקוע.

The Talmud responds: The difference between them is that according to R' Ḥiyya bar Yosef, if the water is raised at any point throughout the night, it is disqualified for sanctification prior to the day’s services. According to R' Yoḥanan, the water is not disqualified unless the Basin is raised at the break of dawn, but the rabbis issued a rabbinic decree of sinking the Basin into its pit all night ab initio, so that it would not be raised at dawn accidentally.

והאמר רבי יוחנן: קידש ידיו לתרומת הדשן ⁠– למחר אין צריך לקדש, שכבר קידש מתחילת עבודה.

The Talmud asks: Does R' Yoḥanan really hold that the Basin must be sunk all night? But doesn’t R' Yoḥanan say: If a priest sanctified his hands and feet in preparation for the removal of the ashes before dawn, then the next day he does not need to sanctify them again, as he already sanctified them at the beginning of the day’s service? Evidently, the Basin is raised before dawn.

בשלמא לרבא, דמוקים לההיא כרבי אלעזר ברבי שמעון ⁠– הא כרבי.

The Talmud elaborates: Granted, according to Rava, who interprets that statement in accordance with the opinion of R' Elazar, son of R' Shimon, that sanctification of the hands and feet is not disqualified overnight (see 20a), this statement that the Basin must be sunk all night is in accordance with the opinion of R' Yehuda HaNasi, who holds that sanctification of the hands and feet is disqualified at dawn. Accordingly, he holds that the water in the Basin is disqualified as well.

אלא לאביי דמוקים לה כרבי ⁠– קשיא דרבי אדרבי: מאי שנא התם דמשקע ליה, ומאי שנא הכא דלא משקע ליה?

But according to Abaye, who interprets that statement in accordance with the opinion of R' Yehuda HaNasi, the first statement of R' Yehuda HaNasi is difficult to reconcile with this other statement of R' Yehuda HaNasi: What is different there that one must sink the Basin all night, and what is different here, with regard to sanctification prior to removal of the ashes, that one does not need to sink it?