Sahih Al Bukhari (146)

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، قَالَ: حَدَّثَنَا اللَّيْثُ، قَالَ: حَدَّثَنِي عُقَيْلٌ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ أَزْوَاجَ النَّبِيّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كُنَّ يَخْرُجْنَ بِاللَّيْلِ إِذَا تَبَرَّزْنَ إِلَى المَنَاصِعِ وَهُوَ صَعِيدٌ أَفْيَحُ "فَكَانَ عُمَرُ يَقُولُ لِلنَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: احْجُبْ نِسَاءَكَ، فَلَمْ يَكُنْ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَفْعَلُ"، فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ، زَوْجُ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، لَيْلَةً مِنَ اللَّيَالِي عِشَاءً، وَكَانَتِ امْرَأَةً طَوِيلَةً، فَنَادَاهَا عُمَرُ: أَلاَ قَدْ عَرَفْنَاكِ يَا سَوْدَةُ، حِرْصًا عَلَى أَنْ يَنْزِلَ الحِجَابُ، فَأَنْزَلَ اللَّهُ آيَةَ الحِجَابِ

`A'ishah (may Allah be pleased with her) narrated that the wives of the Prophet (peace be upon him) 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 (peace be upon him): "Let your wives be veiled," but Allah’s Messenger (peace be upon him) did not do so. One night Sawdah bint Zam`ah, the wife of the Prophet (peace be upon him), went out at `Isha’ time and she was a tall lady. `Umar addressed her saying: "I have recognized you, O Sawdah." He said so, as he desired eagerly that the Ayahs of Hijab (the observing of veils by the Muslim women) may be revealed. So Allah revealed the Ayahs of "Hijab" (A complete body cover excluding the eyes).