Musnad Ahmad 1156:66

'Abd al-Rahman ibn Mahdi narrated to us, 'Abd Allah ibn al-Mubarak narrated to us, from Yunus ibn Yazid, from al-Zuhri, that Nabhan narrated to him, that Umm Salamah narrated to him, she said: I was with the Messenger of Allah, peace and blessings of Allah be upon him, and Maymunah, when Ibn Umm Maktum approached until he entered upon him, and that was after we had been commanded to observe the hijab. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Observe the hijab from him." We said: "O Messenger of Allah, is he not blind, not seeing us and not knowing us?" He said: "Are you both blind? Do you not see him?"