Cows have been considered sacred throughout Hindu tradition, but there are scriptural references to their importance. In the Mahabharata (the great Indian Epic), Bhishma (a great patriarch) says:

No sacrifice can be performed without the aid of curds and ghee (clarified butter). The very character of sacrifice which sacrifices have, depends upon ghee. Hence ghee (or, the cow from which it is produced) is regarded as the very root of sacrifice.

Cows have been said to be the limbs of sacrifice. They represent sacrifice itself. Without them, there can be no sacrifice. With their milk and the Havi produced therefrom, they uphold all creatures by diverse acts. Cows are guileless in their behaviour. From them flow sacrifices and Havya and Kavya, and milk and curds and ghee. hence cows are sacred.

The Mahabharata, strictly speaking, is not revealed scripture (sruti), but it is smrti (remembered scripture), which is based upon or inspired by sruti.