On the 25th of Kislev are the days of Chanukah, which are eight. These were appointed a Festival with Hallel [prayers of praise] and thanksgiving. (Shabbat 21b, Babylonian Talmud)
Our rabbis taught the rule of Chanukah: on the first day one [candle] is lit, and thereafter they are progressively increased... [because] we increase in sanctity but do not reduce. (Shabbat 21b, Babylonian Talmud)
They entered the sanctuary and found only one jar of ritually pure oil that was sufficient to burn only for one day, but they lit the lights of the Menorah from it for eight days, till they pressed olives and extracted additional pure oil.” (Rambam, Hilchot Chanukah, Chapter 3)