Referencia sobre Exodo 31:19

Rabbi Jose followed in the footsteps of Rabbi Akiba in the matter of expounding every saying of the Torah which might have seemed to be superfluous. Thus he explained the verse “but my Sabbaths you shall observe” (Ex. 31:13) to indicate that certain labors may be performed on the Sabbath such as work in the Temple or in order to save a human life. This interpretation he based on the word ״אך״ in the Hebrew text.⁸⁾תוספתא שבת פרק ט״ז.