Musannaf Ibn Abi Shayba 28:1060

Abu Usama narrated to us, from Hisham, who said: A man narrated to me, from Abu Salama, from Aisha, who said: We went out with the Messenger of Allah (may Allah bless him and grant him peace) on a journey, and we stopped at a place. The Messenger of Allah (may Allah bless him and grant him peace) said: "Come, so that I may race you." She said: So I raced him, and I beat him. Then I went out with him after that on another journey, and we stopped at a place. He said: "Come, so that I may race you." She said: So he beat me, and he struck between my shoulders and said: "This is for that."