Nafi' bin Jubayr (may Allah have mercy on him) narrates from his father, Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him), who said: the Prophet (peace and blessings be upon him) said: "The son of a people's sister is one of them."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1563