Narrated 'Aisha:

(the wife of the Prophet) 'Umar bin Al-Khattab used to say to Allah's Apostle "Let your wives be veiled" But he did not do so. The wives of the Prophet used to go out to answer the call of nature at night only at Al-Manasi.' Once Sauda, the daughter of Zam'a went out and she was a tall woman. 'Umar bin Al-Khattab saw her while he was in a gathering, and said, "I have recognized you, O Sauda!" He ('Umar) said so as he was anxious for some Divine orders regarding the veil (the veiling of women.) So Allah revealed the Verse of veiling. (Al-Hijab; a complete body cover excluding the eyes). (See Hadith No. 148, Vol. 1)

حَدَّثَنَا إِسْحَاقُ، أَخْبَرَنَا يَعْقُوبُ، حَدَّثَنَا أَبِي، عَنْ صَالِحٍ، عَنِ ابْنِ شِهَابٍ، قَالَ أَخْبَرَنِي عُرْوَةُ بْنُ الزُّبَيْرِ، أَنَّ عَائِشَةَ ـ رضى الله عنها ـ زَوْجَ النَّبِيِّ ﷺ قَالَتْ كَانَ عُمَرُ بْنُ الْخَطَّابِ يَقُولُ لِرَسُولِ اللَّهِ ﷺ احْجُبْ نِسَاءَكَ. قَالَتْ فَلَمْ يَفْعَلْ، وَكَانَ أَزْوَاجُ النَّبِيِّ ﷺ يَخْرُجْنَ لَيْلاً إِلَى لَيْلٍ قِبَلَ الْمَنَاصِعِ، خَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ، وَكَانَتِ امْرَأَةً طَوِيلَةً فَرَآهَا عُمَرُ بْنُ الْخَطَّابِ وَهْوَ فِي الْمَجْلِسِ فَقَالَ عَرَفْتُكِ يَا سَوْدَةُ. حِرْصًا عَلَى أَنْ يُنْزَلَ الْحِجَابُ. قَالَتْ فَأَنْزَلَ اللَّهُ عَزَّ وَجَلَّ آيَةَ الْحِجَابِ.