Musannaf Ibn Abi Shayba 1:630

Khalid ibn Makhlad narrated to us, from Musa ibn Ya'qub al-Hadrami, who said: Ibn Abi 'Ubaydah ibn 'Abd Allah ibn Zam'ah informed me, from his father, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "When you drink milk, then rinse your mouths with it, for it has fatness."