Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 247
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 247, with English translation by Maimonides (Rambam).
להציל הנרדף ואפילו בנפש הרודף, שנאמר: "וקצותה, את כפה" (דברים כה,יב).
To save a person who is being pursued even if it is necessary to kill the pursuer, as [Deuteronomy 25:12] states: "And you shall cut off her hand...."