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  

The Prophet ﷺ said to his wives, "You are allowed to go out to answer the call of nature. "  

عَنِ النَّبِيِّ ﷺ قَالَ قَدْ أُذِنَ أَنْ تَخْرُجْنَ فِي حَاجَتِكُنَّ قَالَ هِشَامٌ يَعْنِي الْبَرَازَ