Musnad Abu Ya'la al-Mawsili 106:25

Uthman ibn Abi Shaybah narrated to us, Waki’ narrated to us, from Isra’il, from Abu Ishaq, from al-Bara’: That the Prophet, may Allah bless him and grant him peace, when he met the polytheists on the day of Hunayn, he dismounted from his mule and walked on foot.