I believe that the Talmud is the first place where the notion of Olam Haba - The world to come - is mentioned. This question explains why it is not mentioned in the Torah but doesn't quite explain how could such a major facet of Judaism could have such inexplict evidence.
Where does the idea originate from? Even if the Talmud is the first place this is mentioned, surely they must have derived the idea from somewhere, I assume.
In Shmuel 1 28 the whole chapter describes how Shmuel was dead and brought back to this world by the necromancer woman using "ov". In Passuk 3 it describes Shmuel as being dead. Passuk 15 then says:
ויאמר שמואל אל שאול למה הרגזתני להעלות אתי ויאמר שאול צר לי מאד ופלשתים נל -
Shmuel said to Shaul "Why did you anger me by reviving me"
This clearly implies that his soul was enjoying itself in Olam haneshamos and it greatly distressed him to suddenly be summoned back to this world in a much more worst off place.
After clearly telling Shaul what was going on in the past and what was going to happen in the future a feat that only concious beings have the ability to do (you dont find sticks or stones that describe whats going on), in Passuk 19 Shmuel then says:
ומחר אתה ובניך עמי
"Tomorrow you and your sons will join me"
If it is death forevever-more Shmuel describes he could have been more blunt by saying ומחר אתם מתים- "Tomorrow you will die."
Indeed the Gemora Brachos 12b only relates what is seemingly obvious in the Context that they would join Shmuel within his boundaries in the world to come, as they weren't physically buried anywhere near each other Shaul and sons were buried in יביש גלעד (Shmuel 1 31,12) whereas Shmuel was buried in רָמָ֖ה (Shmuuel 1 28,3) and therefore they must have been "together" in another realm in Olam haneshamos.
