Musnad Ahmad 25:1158

Hammad ibn Khalid narrated to us, Ibn Abi Dhi’b narrated to us, from Shu’bah, from Ibn Abbas, who said: I and al-Fadl passed by on a female donkey, while the Messenger of Allah (peace and blessings of Allah be upon him) was praying with the people in an open space of land. We dismounted and entered [the prayer] with him, and he did not say anything to us regarding that.