Sahih Muslim Book 26, Hadith Number 5400.

Chapter : It is prohibited to sit with a strange lady in privacy or to enter her house when she is alone.

‘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.