Breast milk is the milk produced by the breasts (or mammary glands) of a human female to feed a child. Milk is the primary source of nutrition for newborns before they are able to eat and digest other foods; older infants and toddlers may continue to be breastfed, in combination with other foods from six months of age when solid foods should be introduced.
Of course, this is not something of correct behaviour normally accepted by sensible people.
However, as a fiqhi matter, some jurists have deemed it as prohibited, and some have said it is of obligatory precaution not to drink one's wife's milk.
And Allah knows best.