The wives of the Prophet ﷺ used to go to Al-Manasi, a vast open place (near Baqiʿ at Medina) to answer the call of nature at night. ʿUmar used to say to the Prophet ﷺ "Let your wives be veiled," but Allah's Apostle did not do so. One night Sauda bint Zamʿa the wife of the Prophet ﷺ went out at ʿIsha' time and she was a tall lady. ʿUmar 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).  

أَنَّ أَزْوَاجَ النَّبِيِّ ﷺ كُنَّ يَخْرُجْنَ بِاللَّيْلِ إِذَا تَبَرَّزْنَ إِلَى الْمَنَاصِعِ وَهُوَ صَعِيدٌ أَفْيَحُ فَكَانَ عُمَرُ يَقُولُ لِلنَّبِيِّ ﷺ احْجُبْ نِسَاءَكَ فَلَمْ يَكُنْ رَسُولُ اللَّهِ ﷺ يَفْعَلُ فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ زَوْجُ النَّبِيِّ ﷺ لَيْلَةً مِنَ اللَّيَالِي عِشَاءً وَكَانَتِ امْرَأَةً طَوِيلَةً فَنَادَاهَا عُمَرُ أَلاَ قَدْ عَرَفْنَاكِ يَا سَوْدَةُ حِرْصًا عَلَى أَنْ يَنْزِلَ الْحِجَابُ فَأَنْزَلَ اللَّهُ آيَةَ الْحِجَابِ  

Messenger of Allah, ask your ladies to observe veil, but Messenger of Allah ﷺ did not do that. So there went out Sauda, daughter of Zarn'a, the wife of Messenger of Allah ﷺ, during one of the nights when it was dark. She was a tall statured lady. 'Umar called her saying: Sauda, we recognise you. (He did this with the hope that the verses pertaining to veil would be revealed.) 'A'isha said: Allah, the Exalted and Glorious, then revealed the verses pertaining to veil.  

أَنَّ أَزْوَاجَ رَسُولِ اللَّهِ ﷺ كُنَّ يَخْرُجْنَ بِاللَّيْلِ إِذَا تَبَرَّزْنَ إِلَى الْمَنَاصِعِ وَهُوَ صَعِيدٌ أَفْيَحُ وَكَانَ عُمَرُ بْنُ الْخَطَّابِ يَقُولُ لِرَسُولِ اللَّهِ ﷺ احْجُبْ نِسَاءَكَ فَلَمْ يَكُنْ رَسُولُ اللَّهِ ﷺ يَفْعَلُ فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ زَوْجُ النَّبِيِّ ﷺ لَيْلَةً مِنَ اللَّيَالِي عِشَاءً وَكَانَتِ امْرَأَةً طَوِيلَةً فَنَادَاهَا عُمَرُ أَلاَ قَدْ عَرَفْنَاكِ يَا سَوْدَةُ حِرْصًا عَلَى أَنْ يُنْزِلَ الْحِجَابَ قَالَتْ عَائِشَةُ فَأَنْزَلَ اللَّهُ ﷻ الْحِجَابَ  

Messenger of Allah, ask your ladies to observe veil, but Messenger of Allah ﷺ did not do that. So there went out Sauda, daughter of Zarn'a, the wife of Messenger of Allah ﷺ, during one of the nights when it was dark. She was a tall statured lady. 'Umar called her saying: Sauda, we recognise you. (He did this with the hope that the verses pertaining to veil would be revealed.) 'A'isha said: Allah, the Exalted and Glorious, then revealed the verses pertaining to veil. (Using translation from Muslim 2170d)  

أَنَّ أَزْوَاجَ النَّبِيِّ ﷺ كُنَّ يَخْرُجْنَ بِاللَّيْلِ إِذَا تَبَرَّزْنَ إِلَى الْمَنَاصِعِ وَهُوَ صَعِيدٌ أَفْيَحُ وَكَانَ عُمَرُ بْنُ الْخَطَّابِ يَقُولُ لِرَسُولِ اللهِ ﷺ احْجُبْ نِسَاءَكَ فَلَمْ يَكُنْ رَسُولُ اللهِ ﷺ يَفْعَلُ فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ زَوْجُ النَّبِيِّ ﷺ لَيْلَةً مِنَ اللَّيَالِي عِشَاءً وَكَانَتْ امْرَأَةً طَوِيلَةً فَنَادَاهَا عُمَرُ أَلَا قَدْ عَرَفْنَاكِ يَا سَوْدَةُ حِرْصًا عَلَى أَنْ يُنْزَلَ الْحِجَابُ قَالَتْ عَائِشَةُ فَأُنْزِلَ الْحِجَابُ  

[Machine] 'Omar bin Al-Khattab used to say to the Messenger of Allah ﷺ, "Cover your wives." She (one of the wives) said, "But the wives of the Messenger of Allah ﷺ go out at night to the Al-Manasi (the places of relieving oneself).” So, Saudah bint Zam'ah, a tall woman, went out and 'Umar recognized her while he was in the mosque. He said, "I have recognized you, O Saudah, because of your keenness for the veil to be revealed." Allah ﷻ then revealed the verse of the veil.'  

كَانَ عُمَرُ بْنُ الْخَطَّابِ يَقُولُ لِرَسُولِ اللهِ ﷺاحْجُبْ نِسَاءَكَ قَالَتْ فَلَمْ يَفْعَلْ قَالَتْ وَكَانَ أَزْوَاجُ رَسُولِ اللهِ ﷺ يَخْرُجْنَ لَيْلًا إِلَى لَيْلٍ قِبَلَ الْمَنَاصِعِ فَخَرَجَتْ سَوْدَةُ بِنْتُ زَمْعَةَ وَكَانَتْ امْرَأَةً طَوِيلَةً فَرَآهَا عُمَرُ وَهُوَ فِي الْمَسْجِدِ فَقَالَ قَدْ عَرَفْتُكِ يَا سَوْدَةُ حِرْصًا عَلَى أَنْ يُنْزَلَ الْحِجَابُ قَالَتْ فَأَنْزَلَ اللهُ ﷻ الْحِجَابَ