Sayyidah Aishah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), said that the Messenger of Allah (peace and blessings be upon him) said: "Whatever becomes unlawful through birth becomes unlawful through breastfeeding."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12608