Musnad Ahmad 5:564

Abdullah narrated to us, Ibrahim ibn al-Hajjaj narrated to me, Hammad ibn Salamah narrated to us, from Ata ibn al-Sa’ib, from Zadhan: That Ali ibn Abi Talib drank while standing, so the people looked at him and disapproved of that from him. Ali said: "What are you looking at? If I drink while standing, I have seen the Messenger of Allah, may Allah bless him and grant him peace, drink while standing. And if I drink while sitting, I have seen the Messenger of Allah, may Allah bless him and grant him peace, drink while sitting."