Wen our patriarchs passed on it says that god gathered him to his people.
Rambam (author of ikkarei emuna) has a book shaar hagilgulim, gates of gilgulim, – hebrew- you can start there.
there are a wealth of sources for these things all over the place. if i may suggest– ask this question to aish .com (ask the rabbi), you cant go wrong with them. you can ask them anything, and they dont dissapoint, (thats their job;)
good question though. keep asking. only success in all your endeavors:)