Yahya related to me from Malik from Ibn Shihab that he said,"Suckling however little or much, makes haram. Kinship by sucklingmakes men mahram."Yahya said that he had heard Malik say,"Suckling, however little or much when it is in the first two years,makes haram. As for what is after the first two years, little or much,it does not make anything haram. It is like food."
USC-MSA web (English) reference: Book 30, Hadith 11
Arabic reference: Book 30, Hadith 1286