Musnad Ahmad 26:30

Sufyan narrated to us, from Jami’, from Abu Wa’il, from ‘Abdullah, from the Prophet (peace and blessings of Allah be upon him): "No servant withholds the Zakat of his wealth except that a bald-headed serpent is made for him, which follows him. He flees from it, and it follows him, and it says: 'I am your treasure.'" Then ‘Abdullah recited its confirmation in the Book of Allah: {They will be encircled with that which they withheld on the Day of Resurrection} [Al ‘Imran: 180]. Sufyan said once: "It will be encircled around his neck."