It is narrated from Amr bin Shuraid that Ibn Abbas (may Allah be pleased with them) was asked: "A man has two wives. One of them breastfed a boy belonging to someone, and the other breastfed a girl belonging to someone else. Is the marriage between that boy and girl permissible?" He replied: "No. Because the man who provided the semen (from whom the milk of both women was produced) is the same."
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1176
Hadith Takhrijاخرجه الترمذى الرضاع، باب ماجاء في لبن الفحل (1149) - ومالك في الموطا الرضاع، باب رضاعة الصغير، والبيهقي : (453/7)۔