Musnad Ahmad 1153:281

Ibn Numayr narrated to us, Hisham narrated to us, from his father, from Aisha, who said: Sawda went out for her need at night after the hijab had been imposed upon them. She said: She was a woman who towered over the women, large-bodied. Umar encountered her and saw her, so he called out to her: "O Sawda, by Allah, you do not hide from us when you go out, so look at how you go out, or what you do." She turned back and returned to the Messenger of Allah (peace and blessings of Allah be upon him) while he was eating dinner. She informed him of what Umar had said to her, and there was a bone with meat on it in his hand. Then revelation came to him, and then it was lifted from him while the bone was still in his hand. He said: "It has been permitted for you to go out for your needs."