Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 97
Narrated 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. "