Musnad Ahmad 1153:2258

'Umar Abu Hafs al-Mu'ayti narrated to us, he said: Hisham narrated to us, from his father, from 'Aishah, she said: I went out with the Prophet, peace and blessings of Allah be upon him, on one of his journeys while I was a young girl, neither having gained weight nor become heavy. He said to the people: "Go ahead," so they went ahead. Then he said to me: "Come, so that I may race you." So I raced him and I beat him, and he remained silent about me. Until, when I had gained weight and become heavy and had forgotten, I went out with him on one of his journeys. He said to the people: "Go ahead," so they went ahead. Then he said: "Come, so that I may race you." So I raced him, and he beat me. He began to laugh, and he was saying: "This is for that."