It is narrated from Hazrat Aisha (may Allah be pleased with her) that the pure wives of the Prophet (peace and blessings be upon him) used to go out to open fields at night to relieve themselves. Once, Hazrat Sawda went out to relieve herself. Since she was tall and had a heavy build (people would recognize her), on the way she met Hazrat Umar (may Allah be pleased with him). He saw Hazrat Sawda and called out to her from a distance, "Sawda! We have recognized you," and he said this hoping that the command of hijab would be revealed. Consequently, the verse of hijab was revealed.