Musnad Ahmad 27:154

Ibn Idris narrated to us, Imran—meaning Ibn Hudair—informed us, and Waki’—the meaning is the same—said: Imran informed us, from Yazid ibn Utarid, he said: Waki’ said: As-Sadusi, Abu al-Bazri, said: I asked Ibn Umar about drinking while standing? He said: "We used to, during the time of the Messenger of Allah, may Allah bless him and grant him peace, drink while standing, and eat while we were walking."