Musnad Ahmad 5:353

Muhammad ibn Fudayl narrated to us, from ‘Ata’ ibn al-Sa’ib, from Maysarah, who said: I saw ‘Ali drinking while standing. He said: So I said to him, "Do you drink while standing?" He replied: "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."