Musnad Ahmad 25:1191

Muhammad ibn Mus'ab narrated to me, Al-Awza'i narrated to us, from Al-Zuhri, from Ubayd Allah ibn Abd Allah, from Ibn Abbas: That the Messenger of Allah (peace and blessings of Allah be upon him) drank milk, then he called for water and rinsed his mouth. And he said: "Indeed, it has fatness."