Rosh Hashanah, Chapter 1, Mishnah 6. This Mishnah shares with us an incident that occurred in connection with the halachah taught in the previous Mishnah, regarding the question of whether everyone may set out to testify about sighting the new moon.
The text of the Mishnah:
"Ma'aseh she'avru yoser me'arba'im zug, ve'ikvan Rabbi Akiva b'Lod" - more than forty pairs of witnesses set out on the road, and Rabbi Akiva detained them in Lod and prevented them from continuing on up to Jerusalem, since there were already more than enough witnesses there.
"Shalach lo Rabban Gamliel: Im me'akev atah es harabim - nimtzeisa machshilan le'asid lavo" - Rabban Gamliel's argument is that one who prevents the many from coming to testify creates a stumbling block for the future: no one will feel any longer that it is incumbent upon him to set out, since he will rely on others to do it - and in the end no one will go at all.
In summary: in this Mishnah we learned of an incident that took place: Rabbi Akiva detained more than forty pairs of witnesses in Lod, and Rabban Gamliel responded to him that holding back the many is liable to cause them to stumble in the future, until no one will be found who will take the trouble to come and testify.