Musnad Abu Ya'la al-Mawsili 42:241

Zuhayr narrated to us, Abd al-Samad ibn Abd al-Warith narrated to us, my father narrated to me, al-Jurayri narrated to us, from Abu Nadra, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) stood by a stream of rainwater while the people were fasting on a hot day, and they were on foot, and the Messenger of Allah (may Allah bless him and grant him peace) was on his mule. He said, "Drink, O people." They said, "Shall we drink, O Messenger of Allah?" He said, "I am not like you; I am in a better state than you, for I am riding." He said, but they refused. He said, so the Prophet of Allah (may Allah bless him and grant him peace) folded his thigh and dismounted, then he drank, and the people drank, and he had not intended to drink it.