From Sahih Muslim (Hadith 5671):
A'isha reported that the wives of Allah's Messenger (ﷺ) 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 (ﷺ) did not do that. So there went out Sauda, daughter of Zarn'a, the wife of Allah's Messenger (ﷺ), 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
Reference: Muslim 5671