(111) باب لاَ يَخْلُوَنَّ رَجُلٌ بِامْرَأَةٍ إِلاَّ ذُو مَحْرَمٍ، وَالدُّخُولُ عَلَى الْمُغِيبَةِ
Narrated `Uqba bin 'Amir:
Allah's Messenger (ﷺ) 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 (ﷺ) replied: The in-laws of the wife are death itself.
Reference: Sahih al-Bukhari 5232
In-book reference: Book 67, Hadith 165
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 62, Hadith 159