Hayyan bin Umayr (may Allah have mercy on him) said that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: "Seven relations are forbidden because of marriage and seven because of lineage, and nursing also makes forbidden those relations that lineage makes forbidden."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13900