Musnad Ahmad 31:433

'Abd al-Samad narrated to us, my father narrated to me, al-Jurayri narrated to us, from Abu Nadra, from Abu Sa'id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came upon a river from the sky while the people were fasting, on a hot day, traveling on foot, and the Prophet of Allah was on a mule of his. He said: "Drink, O people." They refused. He said: "I am not like you; I am the easiest of you, and I am a rider." But they refused. He said: So the Messenger of Allah (peace and blessings of Allah be upon him) bent his thigh and dismounted, then he drank and the people drank, even though he had not intended to drink.