Tosefta zu Schemot 31:13

R. Yose said: Whence do we know that saving a life takes precedence over the Sabbath? As it is written. “…you shall observe my Sabbaths.” Is it possible that circumcision and the temple service and saving a life [do not take precedence over the Sabbath]? The Torah teaches “But” - a portion. There are times when you observe the Sabbath and times when you do not observe the Sabbath. R. Elazar says: Circumcision takes precedence over the Sabbath because one is punished for it by karet (excision) after time. And the matter is a kal va’homer. If for one limb of him the Sabbath can be superseded, it is logical that the Sabbath should be superseded for all of him. They said to him: according to your reasoning just as in that case it is certain and not uncertain, so too here it must be certain and not uncertain. R. Akiva said: And with respect to which was the Torah more stringent? With the temple service or with the Sabbath? [The Torah was more stringent] with the temple service than with the Sabbath, for the temple service takes precedence over the Sabbath and the Sabbath does not take precedence over the temple service. And the matter is a kal va’homer - if the temple service takes precedence over the Sabbath and a risk to life takes precedence over it, with regard to the Sabbath, which the temple takes precedence over - is it not logical that a risk to human life should supersede it? Behold, you have learned that a risk to life takes precedence over the Sabbath.