Musnad Ahmad 28:164

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I do not fear for my Ummah except for milk, for the devil is between the froth and the pure [milk]."