The website promom.org has a "101 Reasons to Breastfeed Your Child" list. 
What I like most about it is that it's well referenced. Reason #6 says,

"Formula feeding increases baby girls' risk of developing breast cancer in later 
life. 
Women who were formula-fed as infants have higher rates of breast cancer as 
adults. For both premenopausal and postmenopausal breast cancer, women 
who were breastfed as children, even if only for a short time, had a 25% lower 
risk of developing breast cancer than women who were bottle-fed as infants. 

Freudenheim, J. et al. 1994 "Exposure to breast milk in infancy and the risk of 
breast cancer". Epidemiology 5:324-331."

Maybe this is the citation you are looking for?

Best regards,
Michelle Roth, BA, LCCE, CIMI, La Leche League Leader, mom of 4

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