Narrated `Aisha:The wives of the Prophet (ﷺ) 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 (ﷺ) "Let your wives be veiled," but Allah's Apostle did not do so. One night Sauda bint Zam`a the wife of the Prophet (ﷺ) went out at `Isha' time and she was a tall lady. `Umar 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)
Translation note
"The verse of al-Ḥijāb" (āyat al-ḥijāb, literally "the verse of the screen") is not the name of a surah. It refers to 33:53, revealed at the wedding of Zaynab bint Jahsh: "when you ask [the Prophet's wives] for something, ask them from behind a screen." Words in brackets in this translation are the translator's explanation, not part of the Arabic of the hadith.