אורח חיים תנ״חOrach Chayim 458

Se’if 1א׳

נוֹהֲגִים שֶׁלֹּא לָלוּשׁ מַצַּת מִצְוָה בְּעֶרֶב פֶּסַח עַד אַחַר שֵׁשׁ שָׁעוֹת, שֶׁהוּא זְמַן הַקְרָבַת קָרְבַּן פֶּסַח. וּבְי"ד שֶׁחָל לִהְיוֹת בְּשַׁבָּת לָשִׁין בְּעֶרֶב שַׁבָּת אַחַר שֵׁשׁ שָׁעוֹת.

THE COMMANDMENT TO KNEAD THE MATZAH OF THE EVE OF PESACH: ONE SECTION It is customary to not knead Matzot Mitzvah on the eve of Pesach until after the sixth hour, as this was the time when the Pesach offering was offered. When the 14th [of Nissan] falls out on Shabbos, one should knead on the eve of Shabbos after the sixth hour.