Sahih Bukhari

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)