I believe this issue is discussed in the Kuzari 1:109.
BTW, to say there is no mention of Olam Habah in the Torah *at all* is
not fully accurate, while Olam Habah is not explicitly mentioned there
are many allusions to it. Nevertheless, the question is a very good one.
Eliezer C. Abrahamson
176 East 9th St., Lakewood, NJ 08701
(908) 905-6877 e-mail: Abrahamson@juno.com