Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5397
'A'isha reported that the wives of Allah's Messenger (may peace be upon him) used to go out in the cover of night when they went to open fields (in the outskirts of Medina) for easing themselves. 'Umar b Khattab used to say: Allah's Messenger, ask your ladies to observe veil, but Allah's Messenger (may peace be upon him) did not do that. So there went out Sauda, daughter of Zarn'a, the wife of Allah's Messenger (may peace be upon him), during one of the nights when it was dark. She was a tall statured lady. 'Umar called her saying: Sauda, we recognise you. (He did this with the hope that the verses pertaining to veil would be revealed.) 'A'isha said: Allah, the Exalted and Glorious, then revealed the verses pertaining to veil.