Chapter 2: (Al-Baqarah - The Cow)
67. And when Moses said to his people: Surely Allah commands you to sacrifice a cow. They said: Dost thou ridicule us? He said: I seek refuge with Allah from being one of the ignorant.
68. They said: Call on thy Lord for our sake to make it plain to us what she is. (Moses) said: He says, Surely she is a cow neither advanced in age nor too young, of middle age between these (two); so do what you are commanded.
69. They said: Call on thy Lord for our sake to make it clear to us what her colour is. (Moses) said: He says, she is a yellow cow; her colour is intensely yellow delighting the beholders.
70. They said: Call on thy Lord for our sake to make it clear to us what she is, for surely to us the cows are all alike, and if Allah please we shall surely be guided aright.
71. (Moses) said: He says: She is a cow not made submissive to plough the land, nor does she water the tilth, sound, without a blemish in her. They said: Now thou hast brought the truth. So they slaughtered her, though they had not the mind to do (it).
What (you may ask) does a scripture dealing with the sacrifice of an animal have to do with fasting?
You may consider the animal to be your lowerself; animal nature.