I said to Muawiya bin Qurra, "Did you hear Anas say that the Messenger of Allah ﷺ said to Numan bin Muqarrin, 'The son of a people's sister is one of them'?"He replied, "Yes."
حدثنا وكيع
قال قلت لمعاوية بن قرة أسمعت أنسا يقول قال رسول الله ﷺ للنعمان بن مقرن ابن أخت القوم منهم قال نعم
Its chain of transmission is sahih. Bukhari related the hadith 4/221 in "The Merits", chapter "The nephew of a people is one of them", Abu Dawud 4/232 no. 5122 in "Manners", chapter "Tribalism", and Nasai 5/106 no. 2610 in "Marriage", chapter "The nephew of a people is one of them".