סימן קנב סעיף א
אָסוּר לְהִתְיַחֵד עִם שׁוּם אִשָּׁה, בֵּין יַלְדָּה בֵּין זְקֵנָה, בֵּין יִשְֹרָאֵלית בֵּין גּוֹיָה, בֵּין קְרוֹבָתוֹ בֵּין אֵינָהּ קְרוֹבָתוֹ, חוּץ מִן הָאָב שֶׁמֻּתָּר לְהִתְיַחֵד עִם בִּתּוֹ, וְהָאֵם עִם בְּנָהּ, וְהַבַּעַל עִם אִשְׁתּוֹ, אַף עַל פִּי שֶׁהִיא נִדָּה. (וְכַלָּה שֶׁהִיא נִדָּה, עַיֵּן לְקַמָּן סִימָן קנז).
It is forbidden to be secluded with any woman whether young or old, a Jewess or a non-Jewess,1Tur rules that yichud with a Jewish relative with whom relations would be considered incest is a Scriptural prohibition. According to Rambam it is Divrei Kabalah, and the penalty is flogging in the same manner as for violation of Rabbinic ordinances. Yichud with an unmarried girl is prohibited by decree of King David and his court. However, once they have menstruated, they are always in a state of niddah (for unmarried girls are not accustomed to purify themselves from menstrual uncleanliness. See Kitzur Shulchan Aruch 159–162), and therefore yichud with them is prohibited as with relatives. Yichud with a non-Jewess is prohibited by decree of Shamai and Hillel and their courts. See Shulchan Aruch Even Ha’Ezer 22:1,2. whether she is a relative or not a relative, with the exception of a father who may be alone with his daughter;2Or with his granddaughter, great grand etc. (Bach) and a mother with her son,3The issue of yichud with a grandmother requires further deliberation. (Ezer Mikodesh) and a husband with his wife, even though she is menstrually unclean.4You may be alone with your sister temporarily (Rosh). However, Beis Shmuel says the Rambam and Tur possibly disagree. (Concerning a bride who is menstrually unclean see Chapter 157, below).
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.)