Polybius Hist. III 25, 6 in occasion of a treaty stipulated by the fetials between Rome and Carthage; Livy VIII 9, 6 in the formula of the devotio of Decius Mus; Festus s.v. spolia opima, along with Plutarch Marcellus 8, Servius ad Aeneidem VI 860 on the same topic.
Marcellus gained the most prestigious award a Roman general could earn, the spolia opima, for killing the Gallic military leader and king Viridomarus in hand-to-hand combat in 222 BC at the battle of Clastidium.