It is narrated from Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "The relations that are unlawful through birth are unlawful through suckling." Imam Muslim (may Allah have mercy on him) narrated it in his Sahih through the chain of Abu Ma'mar al-Hudhali from Ali bin Hashim.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15606