Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5400
'Uqba b. Amir reported Allah's Messenger (may peace be upon him) as saying: Beware of getting, into the houses and meeting women (in seclusion). A person from the Ansir said: Allah's Messenger, what about husband's brother, whereupon he said: Husband's brother is like death.