Rosh Hashanah 25A

וערבית במערב. אמר רבי יוחנן בן נורי: עדי שקר הם. כשבאו ליבנה קיבלן רבן גמליאל.

and that same day we saw Rosh Chodesh in the evening in the west. R' Yoḥanan ben Nuri said: They are false witnesses, as it is impossible to see Rosh Chodesh so soon after the last sighting of the waning moon. However, when they arrived in Yavne, Rabban Gamliel accepted them as witnesses without concern.

ועוד, באו שנים ואמרו: ראינוהו בזמנו, ובליל עיבורו לא נראה, וקיבלן רבן גמליאל.

And there was another incident in which two witnesses came and said: We saw Rosh Chodesh at its anticipated time, i.e., on the night of the 30th day of the previous month; however, on the following night, i.e., the start of the 31st, which is often the determinant of a full, 30-day month, it was not seen. And nevertheless Rabban Gamliel accepted their testimony and established Rosh Chodesh on the 30th day.

אמר רבי דוסא בן הורכינס: עדי שקר הן, היאך מעידים על האשה שילדה ולמחר כריסה בין שיניה? אמר לו רבי יהושע: רואה אני את דבריך. שלח לו רבן גמליאל: גוזרני עליך שתבא אצלי במקלך ובמעותיך ביום הכפורים שחל להיות בחשבונך.

R' Dosa ben Horkinas disagreed and said: They are false witnesses; how can witnesses testify that a woman gave birth and the next day her belly is between her teeth, i.e., she is obviously still pregnant? If Rosh Chodesh was already visible at its anticipated time, how could it not be seen a day later? R' Yehoshua said to him: I see the logic of your statement; Rosh Chodesh must be established a day later. Upon hearing that R' Yehoshua had challenged his ruling, Rabban Gamliel sent a message to him: I decree against you that you must appear before me with your staff and with your money on the day on which Yom Kippur occurs according to your calculation; according to my calculation, that day is the 11th of Tishrei, the day after Yom Kippur.