סימן קכו סעיף א
מִשֶּׁחָרַב בֵּית הַמִּקְדָּשׁ תִּקְּנוּ חֲכָמֵינוּ זִכְרוֹנָם לִבְרָכָה שֶׁבְּכָל שִׂמְחָה יְהֵא בָּהּ זֵכֶר לַחֻרְבָּן, כְּמוֹ שֶׁכָּתוּב, אִם אֶשְׁכָּחֵךְ יְרוּשָׁלַיִם וְגוֹ' אִם לֹא אַעֲלֶה אֶת יְרוּשָׁלַיִם עַל רֹאשׁ שִׂמְחָתִי. וְגָזְרוּ שֶׁלֹּא יִבְנֶה לוֹ יִשְֹרָאֵל בִּנְיָן מְסֻיָּד וּמְצֻיָּר כְּבִנְיָן הַמְּלָכִים. וְלֹא יְסַיֵּד אֶת כָּל בֵּיתוֹ בְּסִיד, אֶלָּא טָח בֵּיתוֹ בְּטִיט וְסָד בְּסִיד, וּמַנִּיחַ בּוֹ אַמָּה עַל אַמָּה כְּנֶגֶד הַפֶּתַח בְּלֹא סִיד, כְּדֵי לִזְכֹּר הַחֻרְבָּן. וּמַה שֶׁלֹּא נָהֲגוּ כֵן עַתָּה, לֹא יָדַעְנוּ טַעַם בָּרוּר.
After the Beis Hamikdash was destroyed, our Sages, of blessed memory, ordained that on every joyous occasion, there should be a remembrance of the destruction of the Beis Hamikdash, as it is written: "If I forget you, Jerusalem, if I fail to elevate Jerusalem above my foremost joy" (Psalms 137:5). They decreed that no Jew should build a house1It is, however, permitted to build a Beis Midrash or Synagogue without any restrictions. (Biyur Halachah 560:1) for himself that is lime-plastered and decorated like a royal palace.2Tur rules that if a house was merely painted, but not plastered, even though it was decorated, leaving an amah unpainted is sufficient. (see Mishnah Berurah 560:1) Neither should he lime plaster his entire house, but he may plaster the house,3If, however, the house was plastered with mud or clay, but not painted, it is not required to leave an amah unplastered. (Ibid. 560:2) with clay and then lime plaster, it (lit. whitewash it), and he should leave a space of one square amah4See glossary. opposite the entrance5Others have the custom to leave the amah above the door, because the main objective is that the inhabitants of the house see it and be reminded of the destruction, rather than those (strangers who enter the house on occasion). Leaving it opposite the entrance, only serves as a reminder, upon entering the house, but not as a reminder for those who spend time in the house. (Ibid. 560:3) unplastered,6It is not proper to have it painted black. Those who have curtains (wall-paper) around all the walls of the house, should also leave an amah of bare walls. (Ibid. 560:3) as a reminder of the destruction [of the Beis Hamikdash. ] We do not know of any good reason why people do not observe this custom nowadays.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)