Reference for Rosh Hashanah 39:21
מ"ט
Raba said: This teaching which Rabbah B'Samuel learnt follows the 'Others', as it has been taught: 'Others say that between one Pentecost and another and between one New Year and 'another there are always four days [of the week] difference, or, if it was a leap year, five'.<span class="x" onmousemove="('comment',' V. p. 21, nn. 12, 13, and note. For this to happen the months would have to follow the moon strictly.');"><sup>25</sup></span> R'Dimi from Nehardea reports the teaching in the reverse form: 'Witnesses can be intimidated to report [on the thirtieth day] the appearance of the moon which has not appeared in its proper time, in order that the month may be sanctified, but they may not be intimidated to withhold the report of the new moon which has been seen at its proper time in order that the month may be prolonged. What is the reason?