Musnad Ahmad 30:415

'Abd al-A'la narrated to us, from Yunus—meaning Ibn 'Ubayd—from al-Salt ibn Ghalib al-Hujaymi, from Muslim: He asked Abu Hurayrah about drinking while standing. He said: "O son of my brother, I saw the Messenger of Allah (peace and blessings of Allah be upon him) tie his riding camel while it was kneeling, and I was holding its halter or its rein, placing my foot on its foreleg. Then a group of Quraysh came and stood around him. A vessel of milk was brought to the Messenger of Allah (peace and blessings of Allah be upon him), and he drank while he was upon his riding camel. Then he handed it to the one next to him on his right, and he drank while standing, until the whole group had drunk while standing."