Behold, no person should spend the night with a married woman, but only in case he is married to her or he is her Mahram.  

قَالَ رَسُولُ اللَّهِ ﷺ أَلاَ لاَ يَبِيتَنَّ رَجُلٌ عِنْدَ امْرَأَةٍ ثَيِّبٍ إِلاَّ أَنْ يَكُونَ نَاكِحًا أَوْ ذَا مَحْرَمٍ  

Beware of getting, into the houses and meeting women (in seclusion). A person from the Ansar said: Messenger of Allah, what about husband's brother, whereupon he ﷺ said: Husband's brother is like death.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِيَّاكُمْ وَالدُّخُولَ عَلَى النِّسَاءِ فَقَالَ رَجُلٌ مِنَ الأَنْصَارِ يَا رَسُولَ اللَّهِ أَفَرَأَيْتَ الْحَمْوَ قَالَ الْحَمْوُ الْمَوْتُ  

This hadith has been narrated on the authority of Yazid b. Abu Habib with the same chain of transmitters.  

أَنَّ يَزِيدَ بْنَ أَبِي حَبِيبٍ حَدَّثَهُمْ بِهَذَا الإِسْنَادِ مِثْلَهُ  

I heard Laith b. Said as saying: Al-Hamv means the brother of husband or like it from amongst the relatives of the husband, for example, cousin, etc.  

سَمِعْتُ اللَّيْثَ بْنَ سَعْدٍ يَقُولُ الْحَمْوُ أَخُ الزَّوْجِ وَمَا أَشْبَهَهُ مِنْ أَقَارِبِ الزَّوْجِ ابْنُ الْعَمِّ وَنَحْوُهُ  

I did not see but good only (in my wife). Thereupon Messenger of Allah ﷺ said: Verily Allah has made her immune from all this. Then Messenger of Allah ﷺ stood on the pulpit and said: After this day no man should enter the house of another person in his absence, but only when he is accompanied by one person or two persons.  

أَنَّ نَفَرًا مِنْ بَنِي هَاشِمٍ دَخَلُوا عَلَى أَسْمَاءَ بِنْتِ عُمَيْسٍ فَدَخَلَ أَبُو بَكْرٍ الصِّدِّيقُ وَهِيَ تَحْتَهُ يَوْمَئِذٍ فَرَآهُمْ فَكَرِهَ ذَلِكَ فَذَكَرَ ذَلِكَ لِرَسُولِ اللَّهِ ﷺ وَقَالَ لَمْ أَرَ إِلاَّ خَيْرًا فَقَالَ رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ قَدْ بَرَّأَهَا مِنْ ذَلِكَ ثُمَّ قَامَ رَسُولُ اللَّهِ ﷺ عَلَى الْمِنْبَرِ فَقَالَ لاَ يَدْخُلَنَّ رَجُلٌ بَعْدَ يَوْمِي هَذَا عَلَى مُغِيبَةٍ إِلاَّ وَمَعَهُ رَجُلٌ أَوِ اثْنَانِ