Sahih Bukhari Volume 4, Book 56, Hadith 2845

حَدَّثَنَا مُوسَى، حَدَّثَنَا هَمَّامٌ، عَنْ إِسْحَاقَ بْنِ عَبْدِ اللَّهِ، عَنْ أَنَسٍ ـ رضى الله عنه ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم لَمْ يَكُنْ يَدْخُلُ بَيْتًا بِالْمَدِينَةِ غَيْرَ بَيْتِ أُمِّ سُلَيْمٍ، إِلاَّ عَلَى أَزْوَاجِهِ فَقِيلَ لَهُ، فَقَالَ ‏"‏ إِنِّي أَرْحَمُهَا، قُتِلَ أَخُوهَا مَعِي ‏"‏‏

Narrated By Anas : The Prophet used not to enter any house in Medina except the house of Um Sulaim besides those of his wives when he was asked why, he said, "I take pity on her as her brother was killed in my company."