Siman 407

It is a known matter that there is nothing that stands before pikuach nefesh, for one profanes the Shabbat, and all the more so that one is obligated to go outside the techum, even in techumin of twelve mil. And regarding techumin, the sages made an additional enactment for this, that he has in the place where he arrived two thousand amot in every direction, even though he did not acquire shevitah there. And if he is in some city—he is like the people of the city, and he has the entire city and two thousand amot outside of it. Therefore, anyone who went outside the techum with permission, such as a wise midwife who comes to a woman to deliver her child, and so too anything similar to this—they have all the laws that have been explained, and the sages did this so that it would not be a burden upon them.

And not only this, but even if he was going out with permission for the saving of a life, and while he was on the way they told him that the matter for which he is going has already concluded, and he has nothing more to do there, and consequently he remains in his place and is forbidden to desecrate the Shabbat—the sages further gave him two thousand amot in every direction from the place where this was told to him. And if a portion of the techum from which he went out with permission was absorbed within the two thousand amot that he has from his place—he may return to his place, and it is as if he did not go out, for the absorption of techumin is a significant matter, as I wrote at the beginning of siman 405; see there.

All those who go out to save lives from the hand of bandits, or from a river, or from a collapse and the like—they have two thousand amot in every direction from the place where they performed the rescue. And if the hand of the bandits was powerful, and they were afraid to remain for the entire Shabbat day in the place where they rescued—they may return even with their weapons to their place, for any doubt regarding a concern for pikuach nefesh overrides the Shabbat. And as a general rule, those who go out with permission were the witnesses who testified regarding the sighting of the moon, during the time when they would sanctify based on the sighting, as we learned in Rosh Hashanah (23b) and as the Rambam wrote at the end of chapter 27.