Ibn Abbas (may Allah be pleased with them both) said: I heard the Prophet (peace and blessings be upon him) say: "No man should be alone with a woman, and no woman should travel alone without a mahram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13517
The Prophet (peace and blessings be upon him) said: "Beware of entering upon women." A man from the Ansar said: O Messenger of Allah (peace and blessings be upon him)! What do you think about the brother-in-law? He (peace and blessings be upon him) said: "The brother-in-law is death."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13518
A group from Banu Hashim came to Sayyidah Asma' bint Umays (may Allah be pleased with her), and Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) also came in, and Sayyidah Asma' bint Umays (may Allah be pleased with her) was married to Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) at that time. He disliked this and mentioned it to the Prophet (peace and blessings be upon him), and he said: I have seen nothing but good. Then the Prophet (peace and blessings be upon him) said: "Allah Almighty has declared her free of that." Then he (peace and blessings be upon him) stood on the pulpit and said: "After this day, let no man enter upon a woman whose husband is absent unless one or two men are with him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13519
A freed slave of Amr bin al-As (may Allah be pleased with him) narrated that Amr sent him to Sayyiduna Ali (may Allah be pleased with him) to seek permission to enter upon Sayyidah Asma' bint Umays (may Allah be pleased with her). He granted him permission. When he had finished his business, the freed slave asked Amr about that, and he said: The Prophet (peace and blessings be upon him) "forbade us from entering upon women without the permission of their husbands."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13520
It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that he delivered a sermon at a place called Jabiyah and said: The Messenger of Allah (peace and blessings be upon him) stood among us as I stand among you and said: "Treat my Companions well, then those who come after them, then those who come after them. Then lying will become widespread, until a man gives testimony before he is asked for it and swears an oath before he is asked for it. Whoever among you wants the finest, central place in Paradise, let him hold fast to the Jama'ah, for Satan is with the one who is alone and he flees from two. No man should be alone with a woman, for Satan is the third with them. And whoever is pleased by his good deeds and grieved by his bad deeds is a believer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13521