Kohelet Rabbah 7.4

לֵב חֲכָמִים בְּבֵית אֵבֶל, אָמַר רַבִּי יִצְחָק מָשָׁל לְאֶחָד שֶׁאָמַר לְעַבְדּוֹ צֵא וְהָבֵא לִי חֵפֶץ טוֹב, אָמַר לוֹ וּמֵהֵיכָן אֲנִי מוֹצֵא חֵפֶץ טוֹב, אָמַר לוֹ מִשֶּׁתִּמְצָא רֹב בְּנֵי אָדָם הוֹלְכִים, שָׁם חֵפֶץ טוֹב.

“The heart of the wise is in a house of mourning, and the heart of fools in the house of rejoicing” (Ecclesiastes 7:4). “The heart of the wise is in a house of mourning.” Rabbi Yitzḥak said: This is analogous to one who said to his servant: ‘Go out and bring me a fine object.’ He said to him: ‘Where do I find a fine object?’ He said to him: ‘Where you find a multitude of people going, there is a fine object.’