Musnad Abu Ya'la al-Mawsili 110:91

Al-Hakam ibn Musa narrated to us, Hagl narrated to us, he said: I heard al-Awza'i, he said: al-Zuhri narrated to me, from Ubayd Allah ibn Abd Allah ibn Utbah, from Ibn Abbas, that the Messenger of Allah (peace and blessings of Allah be upon him) drank milk, then he rinsed his mouth. Then he said: "Indeed, this has fatness."