Musnad Ahmad 1153:110

Mu'awiyah narrated to us, Abu Ishaq narrated to us, from Hisham ibn 'Urwah, from Abu Salamah ibn 'Abd al-Rahman, who said: 'Aishah informed me that she was with the Prophet, peace and blessings of Allah be upon him, on a journey while she was a young girl. He said to his companions, "Go on ahead," and they went on ahead. Then he said to her, "Come, I will race you," and he mentioned the rest of the hadith.