Musnad Ahmad 31:651

Yazid and Abu al-Nadr narrated to us, from Ibn Abi Dhi'b. Yazid said: Ibn Abi Dhi'b informed us, from al-Zuhri, from Ubayd Allah ibn Abd Allah ibn Utbah, from Abu Sa'id al-Khudri, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the *ikhtinath* (turning inside out) of water skins." Abu al-Nadr said: "That is, to drink from their mouths."