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 ‏"‏ ‏.‏