Siman 285

It is a positive commandment to write the parashah of Shema and "V'haya im shamoa," and to fix them upon the mezuzah of the entrance. For in these two parashiot it is written: "And you shall write them upon the mezuzot of your house and on your gates." And these two parashiot are mutually indispensable. Even a single letter is indispensable, as it is written: "And you shall write them" – that it shall be a finished writing in its entirety. And upon what does one write it, and how is the commandment of its writing performed, will be explained in siman 288 with the help of Heaven.

One must be very careful with this mitzvah, as our sages, of blessed memory, said in Menachot (Menachot 43b): Anyone who has tefillin on his head and on his arm, and tzitzit on his garment, and a mezuzah on his doorway – he is presumed not to sin. And because the mezuzah contains the oneness of the Holy One, Blessed be He, and he will always remember this upon his entering and his exiting – he will place His fear upon his face so as not to sin. And everyone who is careful with it – his days and the days of his children will be lengthened, as it is written: "In order that your days and the days of your children shall be increased." And if he is not careful with it, they will be shortened, for from the "yes" you hear the "no." And so our sages, of blessed memory, expounded in Shabbat (Shabbat 32b): For the sin of mezuzah – his sons and daughters die while they are small. And they expounded this from a verse, see there.

And aside from this, the house is protected by it, as we say in Menachot (Menachot 33b): Come and see that the attribute of the Holy One, blessed be He, is not like the attribute of flesh and blood. The attribute of flesh and blood—the master sits inside, and his servants protect him from without. The attribute of the Holy One, blessed be He, is not so: His servants sit inside, and He protects them from without, as it is said: (Hashem shall guard your going out and your coming in) "Hashem is your guardian, Hashem is your shade upon your right hand." For the mezuzah is on the right side of his entrance, and therefore its placement is in the outermost handbreadth, so that the entire house shall be inside of it and under its protection. Nevertheless, the intent of the one who fulfills it should only be for the sake of the commandment of the Creator, may He be blessed, and the reward comes automatically.

Some say that when a person leaves the house – he should place his hand upon the mezuzah and say "The Lord shall guard my going out and my coming in," and likewise when he enters the house. And there is somewhat of a source for this in the Gemara, chapter "Before their festivals," in the incident of Onkelos, see there (Avodah Zarah 11a). And when he goes out on a journey outside the city, he should place his hand upon the mezuzah and say: "In Your name, Tal, I shall wander," for "Tal" is the gematria of the name Kuzu written on the back of the mezuzah, which are the letters following the name Havayah. And there is one who wrote that he should place his third finger upon the name "Shaddai," and pray to the Lord that He should guard him through this name, and he should kiss this finger (Birkei Yosef).

And with all the greatness of the mitzvah, nevertheless, if his hand cannot reach to buy both tefillin and a mezuzah, we say in the Yerushalmi of Megillah that he should buy tefillin, for a mitzvah that is a personal obligation is preferred. That is to say: that from tefillin the obligation upon him is fixed, whether he is in his house or whether he is on the way. But regarding mezuzah, when he is on the way—he is exempt. And likewise, if he does not have a house—he is exempt from it. And there is one who says that for us, who do not don tefillin except during the time of Kriyat Shema and prayer, if it is possible through borrowing—mezuzah is preferred (Magen Avraham in Orach Chaim siman 38, subsection 4). And beit din does not compel regarding the mitzvah of mezuzah, because it is a mitzvah whose reward is specified alongside it, and as I have written above in siman 240, see there. And one who becomes aware that there is no mezuzah on his doorway—it is forbidden for him to delay even a moment. And he should go to a sofer to buy a mezuzah and fix it immediately. And if it is Shabbat or Yom Tov when it is impossible to fix it, if he has another room—he should not sit in this room. And if he does not have one—it is permitted. And likewise on a weekday when the sofer is not here, and it is impossible to obtain it immediately (Pri Megadim Orach Chaim siman 31). And there are those who say that if the mezuzah is not inside a case—it is forbidden to place his hand upon it and to touch with his hand without an intervention, just as with a Torah (the Gaon Rabbi Akiva Eiger).