Fasting is as old as mankind itself. It was given as a law in paradise. The first commandment Adam received was: "From the tree of the knowledge of good and evil do not eat." Now this command, "do not eat," is the divine law of fasting and temperance. If Eve had fasted from the tree, we would not have to keep this fast now.
St. Basil of Caesarea, Homily 1 on Fasting