Shabbos 21b – After discussing what is the best way to light the Chanukah lights, the Gemara asks the question, “What is Chanukah?” The Gemara then answers that the Rabbis learned that on the 25th of Kislev starts the eight days of Chanukah. We do not eulogize or fast on those days. The Yavanim entered the Holy Temple and defiled all the pure oil they could fine. When the Chashmanaim overpowered the Yavanim, they searched and could only find one jug of oil with the seal of the high priest which contained a one day supply of oil. A miracle happened and the oil lasted for eight days and the next year they set up these days as a holiday to praise and give thanks to Hashem. From this Gemara we see that besides the Chanukah lights we should praise Hashem with the singing of Hallel and thank Hashem with the addition of Al Hanissim.
Shabbos21b [PDF]