Sahih al-Bukhari 3528
Sahih al-Bukhari · Book 61: Virtues and Merits of the Prophet (pbuh) and his Companions · The son of some people's sister is considered as belonging to the same people
Narrated Anas:
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ ، حَدَّثَنَا شُعْبَةُ ، عَنْ قَتَادَةَ ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ دَعَا النَّبِيُّ صلى الله عليه وسلم الأَنْصَارَ فَقَالَ " هَلْ فِيكُمْ أَحَدٌ مِنْ غَيْرِكُمْ ". قَالُوا لاَ، إِلاَّ ابْنُ أُخْتٍ لَنَا. فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " ابْنُ أُخْتِ الْقَوْمِ مِنْهُمْ " .
The Prophet (ﷺ) sent for the Ansar (and when they came), he asked, 'Is there any stranger amongst you?" They said, "No except the son of our sister." Allah's Messenger (ﷺ) said, "The son of the sister of some people belongs to them."