Rosh Hashanah3: 1
רָאוּהוּ בֵית דִּין וְכָל יִשְׂרָאֵל, נֶחְקְרוּ הָעֵדִים, וְלֹא הִסְפִּיקוּ לוֹמַר: ,,מְקֻדָּשׁ”, עַד שֶׁחָשֵׁכָה — הֲרֵי זֶה מְעֻבָּר. רָאוּהוּ בֵית דִּין בִּלְבַד, יַעַמְדוּ שְׁנַיִם וְיָעִידוּ בִפְנֵיהֶם וְיֹאמְרוּ: ,,מְקֻדָּשׁ, מְקֻדָּשׁ”! רָאוּהוּ שְׁלֹשָׁה וְהֵן בֵּית דִּין, יַעַמְדוּ הַשְּׁנַיִם וְיוֹשִׁיבוּ מֵחַבְרֵיהֶם אֵצֶל הַיָּחִיד וְיָעִידוּ בִפְנֵיהֶם, וְיֹאמְרוּ: ,,מְקֻדָּשׁ, מְקֻדָּשׁ”! שֶׁאֵין הַיָּחִיד נֶאֱמָן עַל יְדֵי עַצְמוֹ.
If the beis din and all of Israel saw [the moon], or if the witnesses were interrogated, but they did not manage to say, “It is sanctified,” before it grew dark —- it is full.  If only beis din saw it, two [of them] should stand and testify before the others, and they should say, “It is sanctified, it is sanctified!”  If three saw it and they constitute a beis din, [then] two should stand up and set some of their colleagues beside the single one and testify before them, and say, “It is sanctified, it is sanctified!” For an individual is not trusted alone.