The wives of the Prophet ﷺ used to go out at night to relieve themselves at al-Manasi, an open expanse of ground. Umar would say to the Prophet ﷺ, "Veil your wives," but the Messenger of Allah ﷺ did not do so.One night Sawda bint Zama, the wife of the Prophet ﷺ, went out at the time of the night prayer. She was a tall woman. Umar called out to her, "We have certainly recognized you, Sawda!" He said this hoping that the verse of the veil would be revealed. So Allah revealed the verse of the veil.
أن أزواج النبي ﷺ كن يخرجن بالليل إذاتبرزن إلى المناصع، وهو صعيد أفيح، فكان عمر يقول للنبي ﷺ: احجب نساءك، فلم يكن رسول الله ﷺ يفعل، فخرجت سودة بنت زمعة زوج النبي ﷺ دليل العمل من الليالي عشاء، وكانت امرأة طويلة، فناداها عمر: ألا قد عرفناك يا سودة، حرصا على أن ينزل الحجاب، فأنزل الله آية الحجاب.
Bukhari narrated it in wudu (146), and Muslim in salam (2170:18), both through Layth, who said, "Aqeel narrated to me, from Ibn Shihab, from Urwa, from Ayesha," and he related it. The wording given is Bukhari's, and Muslim's wording is similar.