Musnad Ahmad 1153:1171

Rawh narrated to us, Hammad narrated to us, from Ali ibn Zayd, from Umm Muhammad, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Two serpents are sent upon the disbeliever: one from the direction of his head, and the other from the direction of his feet. They gnaw at him with a gnawing. Whenever they finish, they return [to do so again] until the Day of Resurrection."