Rosh Hashanah, Chapter 1, Mishnah 7. This Mishnah addresses the question of who is fit to testify about sighting the new moon.
The words of the Mishnah:
"Av u'vno she'ra'u es hachodesh - yeilchu" - a father and his son who saw the new moon are obligated to go to beis din and testify.
"Lo shemitztarfin zeh im zeh" - this does not mean that they join together as a single pair of witnesses, for they are relatives, and according to this opinion relatives are disqualified from testifying together.
"Ela she'im yipaseil echad mehem - nitztareif hasheini im acher" - if one of them is found disqualified, the other, who remains fit, can join with a different person who also saw the new moon, someone whom we do not yet know about.
The opinion of Rabbi Shimon:
Rabbi Shimon disagrees and rules: "Av u'vno v'chol hakerovim kesheirim l'eidus hachodesh" - a father and his son and all relatives are fit to join together and serve as witnesses to the sighting of the new moon, unlike all other testimony in the Torah.
His reason, as the Gemara explains, is derived from the verse that the Holy One, Blessed is He, said to Moshe and Aharon: "HaChodesh hazeh lachem" - this is the mitzvah of sanctifying the new month. From here Rabbi Shimon learned that the Torah comes to teach us that their testimony is valid, the two of you alone, even though you are brothers.
The incident with Toviah the doctor:
Rabbi Yose brings an incident that occurred: "Ma'aseh b'Toviah harofei shera'ah es hachodesh biYerushalayim, u'vno v'avdo meshuchrar" - a doctor named Toviah himself saw the new moon in Jerusalem, and so did his son and his freed slave, whose status is like that of a convert.
"V'kiblu haKohanim oso v'es bno u'faslu es avdo" - the kohanim in the Temple accepted his testimony and his son's testimony, since they held like Rabbi Shimon that relatives are fit; but they disqualified his freed slave, because they held that a witness must be of proper lineage, and a freed slave does not have suitable lineage.
"U'v'sheba lifnei beis din - kiblu oso v'es avdo u'faslu es bno" - the great beis din, the Great Sanhedrin, in whose hands the decision rests, disagreed with them: they accepted Toviah himself and his freed slave, and disqualified his son, because they disagreed with Rabbi Shimon and held that relatives are not fit to testify about the new moon.
The opinion of the Sages:
The Sages who disagree with Rabbi Shimon hold that relatives are disqualified from testifying about the new moon. They interpret the verse said to Moshe and Aharon differently: not to teach that relatives are fit, but that the establishment of the month must be done together with the great men of the generation and its leading Sages, and it is they who make the determination.
In summary: In this Mishnah we learned that a father and his son who saw the new moon go to testify, but according to the first opinion they do not join together as a pair; rather, each one may join with a different witness. Rabbi Shimon rules that a father and his son and all relatives are fit for testimony about the new moon, unlike all other testimony in the Torah, based on the verse "HaChodesh hazeh lachem". From the incident with Toviah the doctor a practical dispute emerges: the kohanim accepted him and his son and disqualified his freed slave because of lineage, while the great beis din accepted him and his slave and disqualified his son, following the opinion of the Sages who disqualify relatives and expound that same verse to mean that the establishment of the month is entrusted to the great men of the generation.