Midrash Rabbah מדרש רבה
The collection of midrashim on the Torah and the five scrolls — the principal source of aggada.
Midrash Rabbah is not one book but ten collections under a shared name: one for each book of the Torah and each of the five scrolls. They were compiled at different periods, mostly in the Land of Israel, the oldest being Bereshit Rabbah, roughly contemporary with the completion of the Jerusalem Talmud.
Midrash works differently from Gemara: it does not derive law but opens meaning. It notices an extra letter, an odd word order, a repetition, and builds a story on it. Nearly everything known about Avraham's childhood, about the angels arguing before man was created, about Moshe finding the stray lamb, and about Rachel weeping for her children comes from here.
It does not claim to be a record of events; the sages speak in images, and reading them as reportage is an error both the Rambam and the Maharal warned against.