In modern Jewish literature and folklore the "next world" (עולם הבא) is mentioned multiple times. In contrast, it is not mentioned (AFAIK) at all in the Torah.
So where/when did this start?
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The term is present in the Mishna ; one of the earliest pieces of rabbinic literature (early 3rd century redaction of earlier teachings). E.g. Avot (2: 7), (3: 11), (4: 1), (5: 19).
Various allusions to it are adduced from the Torah, but the lack of explicit reference to it has been noted and discussed throughout the ages. See the answers to this question: Why is there no description of Olam Haba in the Torah?.