Sahih al-Bukhari 6240

'Urwa ibn az-Zubayr related that 'A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "'Umar ibn al-Khattab used to say to the Messenger of Allah, may Allah bless him and grant him peace, 'Veil your wives,"' but the he did not do it. The wives of the Prophet, may Allah bless him and grant him peace, may Allah bless him and grant him peace, used to go out at night to al-Manasi' at night (to defecate). Once Sawda bint Zam'a, the wife of the Prophet, may Allah bless him and grant him peace, went out. She was a tall woman and 'Umar ibn al-Khattab, who was in a gathering, saw her and said, 'We have recognised you, Sawda!' because of his keenness for the Veil to be veiled. So Allah Almighty revealed the Ayat of the Screen."