Musnad Ahmad 30:1812

Qutaybah narrated to us, Layth ibn Sa’d narrated to us, from Ibn ‘Ajlan, from al-Qa’qa’ ibn Hakim, from Abu Salih, from Abu Hurayrah, from the Messenger of Allah (may Allah bless him and grant him peace), that he said: "The treasure of one of them will be on the Day of Resurrection a bald serpent with two black spots, following its owner, while he seeks refuge from it. It will not cease following him until he puts his finger into its mouth."