Musnad Ahmad 5:300

Hashim narrated to us, Shu'bah narrated to us, from 'Asim ibn Kulayb, he said: I heard Abu Burdah narrating from 'Ali: "That the Messenger of Allah, peace and blessings of Allah be upon him, forbade wearing a ring on this [finger] or this [finger]—the middle finger and the index finger." And Jabir—meaning al-Ju'fi—said: "It is the middle finger, there is no doubt about it."