Sawdah went out one night after the verse of hijab had been revealed, in order to relieve herself. She was a large woman, easily recognized by anyone who knew her. Umar bin Khattab saw her and said, "Sawdah, by Allah, you cannot hide yourself from us, so be careful how you go out."She turned back at once. The Messenger of Allah ﷺ was in my house at that moment, eating his evening meal with a bone in his hand. She came in and said, "Messenger of Allah, I went out for some need of mine, and Umar said such and such to me."Then revelation came down upon him, and when it lifted from him, the bone was still in his hand, he had not set it down. He said, "Permission has been granted for you to go out to relieve your needs."
خرجت سودة بعد ما ضرب الحجاب لحاجتها، وكانت امرأة جسيمة لا تخفى على من يعرفها، فرآها عمر بن الخطاب، فقال: يا سودة، أما والله ما تخفين علينا فانظري كيف تخرجين؟ قالت: فانكفأت راجعة ورسول الله ﷺ في بيتي وإنه ليتعشى وفي يده عرق فدخلت، فقالت: يا رسول الله إني خرجت لبعض حاجتي، فقال لي عمر: كذا وكذا قالت: فأوحى الله إليه ثم رفع عنه وإن العرق في يده ما وضعه فقال: إنه قد أذن لكن أن تخرجن لحاجتكن.
Bukhari narrated it in tafsir (4795) and Muslim in salam (2170), both through the route of Abu Usama, from Hisham, from his father, from Ayesha, and mentioned it as above. The wording given is Bukhari's, and Muslim's wording is similar.