Musnad Ahmad 1153:2312

Ya'qub narrated to us, my father narrated to us, from Salih ibn Kaysan, he said: Ibn Shihab said: 'Urwah ibn al-Zubayr informed me that 'Aishah said: 'Umar ibn al-Khattab used to say to the Messenger of Allah, peace and blessings of Allah be upon him: "Veil your women." She said: "But he did not do so." She said: The wives of the Messenger of Allah, peace and blessings of Allah be upon him, used to go out at night to the open fields (al-manasi'). Sawdah bint Zam'ah went out, and she was a tall woman. 'Umar saw her while he was in the mosque and said: "I have recognized you, O Sawdah," out of his eagerness for the verse of the veil to be revealed. She said: "Then Allah, the Mighty and Majestic, revealed the verse of the veil."