Abu Dawud Book 27, Hadith Number 4100.

Chapter : On the meaning of the quranic verse and say to the beleiving woman that they should lower their gaze.

Narated By Umm Salamah, Ummul Mu’minin : I was with the Apostle of Allah (PBUH) while Maymunah was with him. Then Ibn Umm Maktum came. This happened when we were ordered to observe veil (purdah). The Prophet (PBUH) said: Observe veil from him. We asked: Apostle of Allah! is he not blind? He can neither see us nor recognise us. The Prophet (PBUH) said: Are both of you blind? Do you not see him?

Abu Dawud said: This was peculiar to the wives of the Prophet (PBUH). Do you not see that Fatimah daughter of Qays passed her waiting period with Ibn Umm Maktum. The Prophet (PBUH) said to Fatimah daughter of Qays: Pass your waiting period with Ibn Umm Maktum, for he is a blind man. You can put off your clothes with him.