Rosh Hashanah 2:1
Rosh Hashanah2: 1
אִם אֵינָן מַכִּירִין אוֹתוֹ, מְשַׁלְּחִין אַחֵר עִמּוֹ לְהַעִידוֹ. בָּרִאשׁוֹנָה הָיוּ מְקַבְּלִין עֵדוּת הַחֹדֶשׁ מִכָּל אָדָם; מִשֶּׁקִּלְקְלוּ הַמִּינִין, הִתְקִינוּ שֶׁלֹּא יְהוּ מְקַבְּלִין אֶלָּא מִן הַמַּכִּירִים.
If they do not know him, they send another [pair of witnesses] with him to testify about him.  Originally they accepted testimony about the new moon from anyone, [but] after the heretics did harm, they enacted that they should not accept [testimony] except from known people.