Pesachim 4a
פסחים ד׳ א — ד"ה וְהַשְׁתָּא דְּקַיְימָא לַן דִּלְכוּלֵּי עָלְמָא ״אוֹר״
Pesachim 4a:5
The search for leaven is conducted in the evening because that is when people are at home and can properly perform it, and lamplight is suitable for conducting the search at night.
וְהַשְׁתָּא דְּקַיְימָא לַן דִּלְכוּלֵּי עָלְמָא ״אוֹר״ אוּרְתָּא הוּא, מִכְּדֵי בֵּין לְרַבִּי יְהוּדָה וּבֵין לְרַבִּי מֵאִיר — חָמֵץ אֵינוֹ אָסוּר אֶלָּא מִשֵּׁשׁ שָׁעוֹת וּלְמַעְלָה, וְנִבְדּוֹק בְּשֵׁית. אָמַר רַב נַחְמָן בַּר יִצְחָק: בְּשָׁעָה שֶׁבְּנֵי אָדָם מְצוּיִין בְּבָתֵּיהֶם, וְאוֹר הַנֵּר יָפֶה לִבְדִיקָה.
The Gemara returns to the issue of the search for leaven. And now that we maintain that everyone agrees the word or in the mishna is evening, consider the following: After all, both according to the opinion of Rabbi Yehuda and according to that of Rabbi Meir, who disagree with regard to the deadline decreed by the Sages to remove all leaven, it is prohibited to derive benefit from leavened bread by Torah law only from the sixth hour of the day and onward. And if so, let us search for leaven at six hours of the day, and eliminate the leaven at that point. The Gemara cites an answer to its initial question of why the search for leaven is not conducted in the morning. Rav Naḥman bar Yitzḥak said: One searches for leaven in the evening as it is a time when people are found in their homes, and they have the opportunity to perform the search. And furthermore, the light of the lamp is favorable for conducting a search specifically at night. As the search is conducted with a lamp, it is preferable to search at night.