Reference for Rosh Hashanah 41:4
רבי אייבו בר נגרי ור' חייא בר אבא איקלעו לההוא אתרא דהוה מטו שלוחי ניסן ולא מטו שלוחי תשרי ועבדי חד יומא ולא אמרו להו ולא מידי שמע רבי יוחנן ואיקפד אמר להו לאו אמרי לכו היכא דמטו שלוחי ניסן ולא מטו שלוחי תשרי ליעבדו תרי יומי גזירה ניסן אטו תשרי
R'Aibu B'Nagri and R'Hiyya B'Abba once arrived at a certain place which had been reached by the messengers sent out in Nisan but not by those sent out in Tishri, and though the inhabitants kept only one day [of Passover] they did not reprove them. When R'Johanan heard this he was annoyed and said to them: Did I not tell you that in places which have been reached by the messengers sent out in Nisan but not by those sent out in Tishri they should keep two days, Nisan being included so that no mistake should be made in Tishri? Rabbah was accustomed to fast two days [on the Day of Atonement].<span class="x" onmousemove="('comment',' Being uncertain whether the month of Elul, which normally consisted of twenty-nine days, had not been prolonged by the Beth din in Jerusalem.');"><sup>9</sup></span>