Sahih Bukhari Volume 7, Book 62, Hadith Number 159.

Narrated By ‘Uqba bin ‘Amir : Allah’s Apostle said, “Beware of entering upon the ladies.” A man from the Ansar said, “Allah’s Apostle! What about Al-Hamu the in-laws of the wife (the brothers of her husband or his nephews etc.)?” The Prophet replied: The in-laws of the wife are death itself.