Jami` at-Tirmidhi Book 43, Hadith 49
from the freed slave of 'Amr bin Al-'As that 'Amr bin Al-'As sent him to 'Ali seeking his permission to enter upon Asma bint Umais, so he permitted him. When he was finished from what he needed, the freed slave of 'Amr bin Al-'As asked about that, so he said: "Indeed the Prophet (ﷺ) prohibited us - or - prohibited that we enter upon women, without the permission of their husbands."
to Ibn Shihab, that Umm Salamah narrated to him, that she and Maimunah were with the Messenger of Allah (ﷺ), she said: "So when we were with him, Ibn Umm Maktum came, and he entered upon him, and that was after veiling had been ordered for us. So the Messenger of Allah (ﷺ) said: 'Veil yourselves from him.' So I said: 'O Messenger of Allah! Is he not blind such that he can not see us or recognize us?' So the Messenger of Allah (ﷺ) said: 'Are you two blind such that you can not see him?'"
that the Prophet (ﷺ) said: "I have not left among the people after me, a Fitnah more harmful upon men than women."