Hadith 9:10
Kathir b. ‘Abdallah, on his father’s authority, said that his grand-father reported God's Messenger as saying, “The freedman of a people is one of them, the ally* of a people is one of them, and a sister’s son has blood-relationship to her family.” * Or "close friend". Dawud transmitted it.
وَعَنْ كَثِيرِ بْنِ عَبْدِ اللَّهِ عَنْ أَبِيهِ عَنْ جَدِّهِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَوْلَى الْقَوْمِ مِنْهُمْ وَحَلِيفُ الْقَوْمِ مِنْهُمْ وَابْنُ أُخْتِ الْقَوْمِ مِنْهُمْ» . رَوَاهُ الدَّارمِيّ
Original Reference: Mishkat al-Masabih 3051