Reference for Rosh Hashanah 1:7
אָב וּבְנוֹ שֶׁרָאוּ אֶת הַחֹדֶשׁ, יֵלְכוּ. לֹא שֶׁמִּצְטָרְפִין זֶה עִם זֶה, אֶלָּא שֶׁאִם יִפָּסֵל אֶחָד מֵהֶן, יִצְטָרֵף הַשֵּׁנִי עִם אַחֵר. רַבִּי שִׁמְעוֹן אוֹמֵר, אָב וּבְנוֹ וְכָל הַקְּרוֹבִין, כְּשֵׁרִין לְעֵדוּת הַחֹדֶשׁ. אָמַר רַבִּי יוֹסֵי, מַעֲשֶׂה בְטוֹבִיָּה הָרוֹפֵא, שֶׁרָאָה אֶת הַחֹדֶשׁ בִּירוּשָׁלַיִם, הוּא וּבְנוֹ וְעַבְדּוֹ מְשֻׁחְרָר, וְקִבְּלוּ הַכֹּהֲנִים אוֹתוֹ וְאֶת בְּנוֹ, וּפָסְלוּ אֶת עַבְדּוֹ. וּכְשֶׁבָּאוּ לִפְנֵי בֵית דִּין, קִבְּלוּ אוֹתוֹ וְאֶת עַבְדּוֹ, וּפָסְלוּ אֶת בְּנוֹ:
A father and his son who saw the new moon should go (to testify). Not that they may combine (as witnesses) with each other, but so that if one of them is invalidated, the second can combine with another. R. Shimon says: A father and his son and all kin are kasher for testifying to the (appearance of the) new moon, [the L-rd having said to Moses and Aaron (Exodus 12:2): "This month is to you, etc.", i.e., This testimony to the (appearance of) the new moon is kasher through you, even though you are brothers. And the rabbis who differ say: "This testimony is to you, etc.", i.e., This testimony is relegated to you, to the great ones of the generation, such as you. The halachah is not in accordance with R. Shimon.] R. Yossi said: Once Tuvia the physician saw the new moon in Jerusalem — he and his son, and his freed bondsman — and the Cohanim accepted him and his son and invalidated his bondsman; and when they came before beth-din, they accepted him and his bondsman and invalidated his son.