كتاب فضائل الصحابة رضى الله تعالى عنهم
44  
The Book of the Merits of the Companions
(19)
Chapter: The Virtues Of Umm Sulaim, The Mother Of Anas Bin Malik, And Bilal (RA)
(19)
باب مِنْ فَضَائِلِ أُمِّ سُلَيْمٍ أُمِّ أَنَسِ بْنِ مَالِكٍ وَبِلاَلٍ رضى الله عنهما ‏‏
Sahih Muslim 2455

Anas reported that Allah's Apostle (ﷺ) did not enter the house of any woman except that of his wives and that of Umm Sulaim. He used to visit her. It was said to him why it was so, whereupon he said:

I feel great compassion for her. Her brother was killed while he was with me.
حَدَّثَنَا حَسَنٌ الْحُلْوَانِيُّ، حَدَّثَنَا عَمْرُو بْنُ عَاصِمٍ، حَدَّثَنَا هَمَّامٌ، عَنْ إِسْحَاقَ بْنِ عَبْدِ، اللَّهِ عَنْ أَنَسٍ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم لاَ يَدْخُلُ عَلَى أَحَدٍ مِنَ النِّسَاءِ إِلاَّ عَلَى أَزْوَاجِهِ إِلاَّ أُمِّ سُلَيْمٍ فَإِنَّهُ كَانَ يَدْخُلُ عَلَيْهَا فَقِيلَ لَهُ فِي ذَلِكَ فَقَالَ ‏ "‏ إِنِّي أَرْحَمُهَا قُتِلَ أَخُوهَا مَعِي ‏"‏ ‏.‏
Reference : Sahih Muslim 2455
In-book reference : Book 44, Hadith 149
USC-MSA web (English) reference : Book 31, Hadith 6010
  (deprecated numbering scheme)