Srimad Bhagavatam Canto 12, Chapter 03, Text 36

SB 12.3.36

patim tyaksyanti nirdravyam
 bhrtya apy akhilottamam
bhrtyam vipannam patayah
 kaulam gas capayasvinih
 
Translation: 
 
Servants will abandon a master who has lost his wealth, even if that master is a saintly person of exemplary character. Masters will abandon an incapacitated servant, even if that servant has been in the family for generations. Cows will be abandoned or killed when they stop giving milk.
 
Purport: 
 
In India, the cow is considered sacred not because Indian people are primitive worshipers of mythological totems but because Hindus intelligently understand that the cow is a mother. As children, nearly all of us were nourished with cow’s milk, and therefore the cow is one of our mothers. Certainly one’s mother is sacred, and therefore we should not kill the sacred cow.
Srimad Bhagavatam Canto 12, Chapter 03, Text 35
Srimad Bhagavatam Canto 12, Chapter 03, Text 37