Language:
كتاب المناقب
61  
Virtues and Merits of the Prophet (pbuh) and his Companions
(14)
Chapter: The son of some people's sister is considered as belonging to the same people
(14)
باب ابْنُ أُخْتِ الْقَوْمِ وَمَوْلَى الْقَوْمِ مِنْهُمْ
Sahih al-Bukhari 3528

Narrated Anas:

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."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 3528
In-book reference : Book 61, Hadith 38
USC-MSA web (English) reference : Vol. 4, Book 56, Hadith 729
  (deprecated numbering scheme)