Musnad Ahmad 5:561

'Abdullah narrated to us, Wahb ibn Baqiyyah al-Wasiti narrated to me, Khalid ibn 'Abdullah narrated to us, from 'Ata' ibn al-Sa'ib, from Maysarah and Zadhan, they said: 'Ali, may Allah be pleased with him, drank while standing, then he said: "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."