L’Exode 22:1 Midrash : Mekhilta DeRabbi Yishmael

R. Yishmael and R. Elazar b. Azaryah and R. Akiva were once walking on the road, with Levi Hasadar and R. Yishmael the son of R. Elazar b. Azaryah walking behind them, when this question was asked: Whence is it derived that the saving of a life overrides the Sabbath? R. Yishmael responded: It is written (Ibid. 22:1) "If the thief be found breaking in, etc." — If in such an instance, where it is doubtful whether he is coming to steal or to kill, and the spilling of blood defiles the land and causes the Shechinah to depart, he is permitted to kill the thief to save his life — how much more so does the saving of life override the Sabbath!

(Exodus 22:1) "If the thief be found breaking in": This speaks of a doubt (in the owner's mind) as to whether he is breaking in to steal or to kill. You say this, but perhaps the doubt is as to whether he is breaking in to steal or not to steal. Would you say that? If he (broke in) to steal of a certainty and he killed him, he would be liable (for murder), how much more so if there is a doubt as to whether he came to steal! We must thus perforce accept not the second supposition, but the first.