Sahih Bukhari Volume 1, Book 4, Hadith 146

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، قَالَ حَدَّثَنَا اللَّيْثُ، قَالَ حَدَّثَنِي عُقَيْلٌ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ أَزْوَاجَ النَّبِيِّ، صلى الله عليه وسلم كُنَّ يَخْرُجْنَ بِاللَّيْلِ إِذَا تَبَرَّزْنَ إِلَى الْمَنَاصِعِ ـ وَهُوَ صَعِيدٌ أَفْيَحُ ـ فَكَانَ عُمَرُ يَقُولُ لِلنَّبِيِّ صلى الله عليه وسلم احْجُبْ نِسَاءَكَ‏.‏ فَلَمْ يَكُنْ رَسُولُ اللَّهِ صلى الله عليه وسلم يَفْعَلُ، فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ زَوْجُ النَّبِيِّ صلى الله عليه وسلم لَيْلَةً مِنَ اللَّيَالِي عِشَاءً، وَكَانَتِ امْرَأَةً طَوِيلَةً، فَنَادَاهَا عُمَرُ أَلاَ قَدْ عَرَفْنَاكِ يَا سَوْدَةُ‏.‏ حِرْصًا عَلَى أَنْ يَنْزِلَ الْحِجَابُ، فَأَنْزَلَ اللَّهُ آيَةَ الْحِجَابِ

Narrated By 'Aisha : The wives of the Prophet used to go to Al-Manasi, a vast open place (near Baqia 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).