(ב) שִׁמְעוֹן הַצַּדִּיק הָיָה מִשְּׁיָרֵי כְנֶסֶת הַגְּדוֹלָה. הוּא הָיָה אוֹמֵר, עַל שְׁלשָׁה דְבָרִים הָעוֹלָם עוֹמֵד, עַל הַתּוֹרָה וְעַל הָעֲבוֹדָה וְעַל גְּמִילוּת חֲסָדִים:
(2) Shimon the Righteous was one of the last of the men of the great assembly. He used to say: the world stands upon three things: the Torah, the Temple service, and the practice of acts of piety.
The Gemara makes a parenthetical observation. Rabbi Ḥanina says: Conclude from it that the seal of the Holy One, Blessed be God, is truth.
(יח) רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, עַל שְׁלשָׁה דְבָרִים הָעוֹלָם עוֹמֵד, עַל הַדִּין וְעַל הָאֱמֶת וְעַל הַשָּׁלוֹם, שֶׁנֶּאֱמַר (זכריה ח) אֱמֶת וּמִשְׁפַּט שָׁלוֹם שִׁפְטוּ בְּשַׁעֲרֵיכֶם:
(18) Rabban Shimon ben Gamaliel used to say: on three things is the world sustained: On justice, on truth and on peace, as it is said: “execute the judgment of truth and peace in your gates” (Zechariah 8:16).
...Rav Yehuda says that Shmuel says: With regard to these three matters alone, it is normal for Sages to amend their statements and deviate from the truth: With regard to a tractate, if he is asked whether he studied a particular tractate, he may humbly say that he did not, even if he did. And with regard to a bed, if he is asked whether he slept in a particular bed, he may say that he did not, to avoid shame in case some unseemly residue is found on the bed. And he can lie with regard to a host [ushpiza], as one may say that he was not well received by a certain host to prevent everyone from taking advantage of the host’s hospitality. What is the practical difference that emerges from this statement with regard to matters in which Torah scholars deviate from the truth? Mar Zutra says: The practical difference is with regard to returning a lost item on the basis of visual recognition. If we know about him that he alters his statements only with regard to these three matters, we return the lost item to him, but if he alters his statements with regard to other matters, we do not return the lost item to him.
Simple Restatement:
Rav Yehuda said in the name of Shmuel, "In only these three matters is it the practice of the rabbis to deviate in their speech [from the truth]: regarding a section [of rabbinic study], intimacy, and hospitality.

