1. Talmud Eruvin 13
א"ר אבא אמר שמואל שלש שנים נחלקו ב"ש וב"ה הללו אומרים הלכה כמותנו והללו אומרים הלכה כמותנו יצאה בת קול ואמרה אלו ואלו דברי אלקים חיים הן והלכה כב"ה וכי מאחר שאלו ואלו דברי אלקים חיים מפני מה זכו ב"ה לקבוע הלכה כמותן מפני שנוחין ועלובין היו ושונין דבריהן ודברי ב"ש ולא עוד אלא שמקדימין דברי ב"ש לדבריהן
Rabbi Abba said in the name of Shmuel: For three years, the House of Hillel and the House of Shammai argued. One said, 'The halakha is like us,' and the other said, 'The halakha is like us.' A heavenly voice spoke: "These and these are the words of the living God, and the halakha is like the House of Hillel." A question was raised: Since the heavenly voice declared: "Both these and those are the words of the Living God," why did the House of Hillel merit having the halacha established to follow them? It is because the students of Hillel were kind and gracious. They taught their own ideas as well as the ideas from the students of Shammai. Not only that, but made sure to teach Shammai's opinions first.
2. Midrash Psalms 12 (10th Century)
רבי יהושע בן לוי בשם רבי יוסי אומר תינוקות שהיו בימי שאול ושמואל עד שלא הביאו שתי שערות היו דורשין את התורה במ"ט פנים טהור ובמ"ט פנים טמא
Rabbi Yehoshua ben Levi said: Young students in the times of Saul, David and Samuel knew how to study the Torah with forty-nine reasons to rule a matter pure and forty-nine reasons to rule that the same matter was impure.
3. Tosefta Sotah 7:7
אף אתה עשה לבך חדרי חדרים והכניס בו דברי ב"ש ודברי ב"ה דברי המטמאין ודברי המטהרין
Now make yourself a heart of many rooms, and bring into it the words of Beit Shammai and the words of Beit Hillel, the words of those who declare impure and the words of those who declare pure.
4. Rabbi David Hartman (z"l). A Heart of Many Rooms, pp. 21-23
In other words, become a person in whom different opinions can reside together in the very depths of your soul. Become a religious person who can live with ambiguity, who can feel religious conviction and
passion without the need for simplicity and absolute certainty.
…This then is the distinctive legacy of the talmudic interpretive tradition: an understanding of the revelation in which God loves you when you discover ambiguity in God's word. God loves you for finding forty-nine ways to make this pure and forty-nine ways to make it impure. Revelation is not always "pure and simple" but may be rough and complex...The religious personality this system tries to produce is able to interpret situations in multiple ways and to offer cogent arguments for opposite positions and points of view. This orientation reflects a particular kind of religious humility. What has often been portrayed as legalism…is a superficial misrepresentation of the deep joy in study and fascination with the rich complexity of the Torah.