Musnad Ahmad 5:233

'Affan narrated to us, Hammad narrated to us, from 'Ata' ibn al-Sa'ib, from Zadhan, that 'Ali ibn Abi Talib drank while standing. The people looked at him as if they disapproved of it, so he said: "What are you looking at? If I drink while standing, I have seen the Prophet, may Allah bless him and grant him peace, drink while standing. And if I drink while sitting, I have seen the Prophet, may Allah bless him and grant him peace, drink while sitting."