From Sahih Muslim (Hadith 6319):
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
Reference: Muslim 6319