Yevamos, Chapter Twelve, Mishnah 5. In this mishnah we take up the question of how many people must be present at the time of the chalitzah.
The text of the mishnah:
"HaChalitzah bishnayim o bishloshah, v'nimtza echad mehem karov o pasul - chalitzasah pesulah" - the chalitzah was performed before two or before three, and one of them turned out to be a relative or otherwise disqualified, her chalitzah is invalid.
"Bishnayim" - the chalitzah was performed before only two men, that is, a beis din of two.
"O bishloshah" - it was performed before three, but one of them turned out to be a relative, either of the yavam or of the yevamah, or disqualified from testimony and judgment. The result is that only two valid men are present, so the case reverts to a chalitzah performed before two.
According to the first Tanna, chalitzah requires a beis din of three, and therefore this chalitzah is invalid.
The dispute among the Tannaim:
"Rabbi Shimon v'Rabbi Yochanan HaSandlar machshirin" - Rabbi Shimon and Rabbi Yochanan HaSandlar hold that even a chalitzah performed before fewer than three is valid.
The incident in the prison:
The mishnah relates an incident involving a man who performed chalitzah privately with the yevamah in prison, while they were confined there. The Gemara explains that two men saw what took place and were able to testify about it. The case came before Rabbi Akiva, who was also in the prison at that time, and he ruled the chalitzah valid. This serves as proof for the position of Rabbi Shimon and Rabbi Yochanan HaSandlar, that chalitzah does not require a beis din of three in order to be valid.
In summary: in this mishnah we learned the dispute over how many people are needed for chalitzah. According to the first Tanna a beis din of three is required, and if one of them turns out to be a relative or disqualified, her chalitzah is invalid. Rabbi Shimon and Rabbi Yochanan HaSandlar, however, validate it even before fewer than three, and their view emerges from the incident in which Rabbi Akiva ruled the chalitzah in the prison valid.