The Hanukkah oil miracle is discussed in the Talmudic tractate Shabbat.
The passage appears in the Babylonian Talmud, Shabbat 21b. It explains that after the Maccabees reclaimed and rededicated the Temple, they found only one sealed container of ritually pure olive oil. The supply should have burned for one day, but according to the rabbinic account it lasted for eight days, allowing new oil to be prepared.
This discussion is part of a wider legal treatment of Hanukkah lights. The Talmud considers where the lamps should be placed, how many lights should be kindled, and the principle of increasing the number of lights over the festival. The familiar eight-day oil narrative is therefore preserved in a legal-religious discussion rather than in the earliest surviving accounts of the revolt.
A common mix-up is naming Hallel or Megillah. Hallel is a set of Psalms recited on Hanukkah, while Megillah is the tractate associated especially with Purim. Shabbat 21b is the standard citation for the oil story.