It is narrated from Aishah (may Allah be pleased with her) that the wives of the Prophet (peace and blessings be upon him) used to go out at night to an open place to relieve themselves, and Umar bin al-Khattab (may Allah be pleased with him) would say to the Prophet (peace and blessings be upon him): "Command (your wives) to observe hijab," but he (peace and blessings be upon him) did not do so. One night, at the time of Isha, Sawdah bint Zam'ah (may Allah be pleased with her) went out. Umar bin al-Khattab (may Allah be pleased with him) called out and said: "O Sawdah! I have recognised you." He did this hoping that a command concerning hijab would be revealed. Aishah (may Allah be pleased with her) says: Then the verse of hijab was revealed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13505