Komentarz do Rosz ha-Szana 1:6

ארבעים זוג – sets of witnesses one pair after another

Introduction This mishnah illustrates a concept which we mentioned at the end of yesterday’s mishnah-- anything that might lead to witnesses not coming when their testimony is needed is discouraged.

ועכבן ר"ע – for the Jewish court had no need of them

It happened that more than forty pairs of witnesses were on their way [to Jerusalem] and Rabbi Akiva detained them in Lod. Rabban Gamaliel sent to him saying: if you prevent the multitude [from coming to provide testimony] it will turn out that you cause them to stumble in the future. The mishnah describes Rabbi Akiva in Lod holding back witnesses who were about to make their way to Jerusalem to testify that they had seen the new moon. Rabbi Akiva detains the because he reasons that there is no need for so many witnesses to testify; after all, two would seem to be sufficient. Commentators add that this transpired on Shabbat. Rabbi Akiva did not think it was necessary for forty pairs of witnesses to profane Shabbat, when only two were needed, so he held the unneeded pairs back. Rabban Gamaliel rebuked Rabbi Akiva for his actions. Witnesses who were told not to go to Jerusalem to testify would not go next time they see the new moon, and perhaps that time they would actually be needed. This is what he means when he says, “cause them to stumble.”

אתה מכשילן לעתיד לבא – they will be prevented from coming and testifying when they saw that their words were not listened to. And the Halakha is according to Rabban Gamaliel.