The 311th prohibition is that we are forbidden from taking a newlywed man from his home for any duty — military or otherwise — for one year [from his wedding]. For the entire year we exempt him from any responsibilities that would cause him to be absent from home.

The source of this prohibition is G‑d's statement1 (exalted be He), "[When a man takes a new bride, he shall not enter military service] nor shall he be assigned to any duty."

The Gemara says in tractate Sotah,2 "From the phrase, 'he shall not enter military service,' one could think that only military service is prohibited, but that he should be assigned to prepare weapons and supply water and food. The Torah therefore adds, 'nor shall he be assigned to any duty.' The word 'he [be assigned]' teaches that only he may not be assigned, but that others3 may be assigned. But since we could learn it from, 'nor shall he be assigned to any duty,4' why is it written, 'he shall not enter military service'? So that the transgressor be in violation of two prohibitions." We already explained in the Ninth Introductory Principle that not always does the violation of "two prohibitions" constitute two commandments.

You should be aware that the newlywed himself5 is also prohibited from leaving home, i.e., traveling abroad, for the entire year.

The details of this mitzvah are explained in the eighth chapter of tractate Sotah.6