Musnad Ahmad 30:3190

Isma'il narrated to us, he said: Ayyub informed us, from 'Ikrimah, from Abu Hurairah, "That the Messenger of Allah (peace and blessings of Allah be upon him) forbade drinking from the mouth of a waterskin." Ayyub said: "I was informed that a man drank from the mouth of a waterskin, and a snake came out."