Eruvin
Eruvin 13b:10
The Talmud states that the law is established according to Beit Hillel because they were known to be more agreeable and would report not only their own opinions but also those of Beit Shammai.
אָמַר רַבִּי אַבָּא אָמַר שְׁמוּאֵל: שָׁלֹשׁ שָׁנִים נֶחְלְקוּ בֵּית שַׁמַּאי וּבֵית הִלֵּל, הַלָּלוּ אוֹמְרִים: הֲלָכָה כְּמוֹתֵנוּ, וְהַלָּלוּ אוֹמְרִים: הֲלָכָה כְּמוֹתֵנוּ. יָצְאָה בַּת קוֹל וְאָמְרָה: אֵלּוּ וָאֵלּוּ דִּבְרֵי אֱלֹהִים חַיִּים הֵן, וַהֲלָכָה כְּבֵית הִלֵּל. וְכִי מֵאַחַר שֶׁאֵלּוּ וָאֵלּוּ דִּבְרֵי אֱלֹהִים חַיִּים, מִפְּנֵי מָה זָכוּ בֵּית הִלֵּל לִקְבּוֹעַ הֲלָכָה כְּמוֹתָן? מִפְּנֵי שֶׁנּוֹחִין וַעֲלוּבִין הָיוּ, וְשׁוֹנִין דִּבְרֵיהֶן וְדִבְרֵי בֵּית שַׁמַּאי, וְלֹא עוֹד אֶלָּא שֶׁמַּקְדִּימִין דִּבְרֵי בֵּית שַׁמַּאי לְדִבְרֵיהֶן.
Rabbi Abba said that Shmuel said: For three years Beit Shammai and Beit Hillel disagreed. These said: The halakha is in accordance with our opinion, and these said: The halakha is in accordance with our opinion. Ultimately, a Divine Voice emerged and proclaimed: Both these and those are the words of the living God. However, the halakha is in accordance with the opinion of Beit Hillel. The Gemara asks: Since both these and those are the words of the living God, why were Beit Hillel privileged to have the halakha established in accordance with their opinion? The reason is that they were agreeable and forbearing, showing restraint when affronted, and when they taught the halakha they would teach both their own statements and the statements of Beit Shammai. Moreover, when they formulated their teachings and cited a dispute, they prioritized the statements of Beit Shammai to their own statements, in deference to Beit Shammai.