Musnad Abu Ya'la al-Mawsili 244:6

Abu Bakr narrated to us, Ghundar narrated to us, from Shu'bah, from al-Hakam, who said: I heard Dhakwan narrating from a freed slave of 'Amr ibn al-'As, that he ('Amr) sent him to 'Ali to ask permission to enter upon Asma' bint 'Umays, and he (Ali) gave him permission. When he had finished his need, the freed slave asked 'Amr about that. He said: "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) forbade us from entering upon women without the permission of their husbands."