The going out of women for answering the call of nature

Narrated `Aisha:
The wives of the Prophet used to go to Al-Manasi, a vast open place (near Baqi` at Medina) to answer the call of nature at night. `Umar used to say to the Prophet "Let your wives be veiled," but Allah's Apostle did not do so. One night Sauda bint Zam`a the wife of the Prophet went out at `Isha' time and she was a tall lady. `Umar addressed her and said, "I have recognized you, O Sauda." He said so, as he desired eagerly that the verses of Al-Hijab (the observing of veils by the Muslim women) may be revealed. So Allah revealed the verses of "Al-Hijab" (A complete body cover excluding the eyes)

English translation: Vol. 1, Book 4, Hadith 148.