Jovem lapidem jurare → swear by Jupiter the stone. Lewis & Short, Latin Dictionary s.v. Juppiter: "Jovem lapidem jurare, said of one who swore by Jupiter (holding in one hand a knife with which he pierced the sacrificial sow, and in the other hand a stone)." S.v. lapis: "A statue: Jovem lapidem jurare, the statue of Jupiter at the Capitol."
Δία λίθον ὀμνύναι (Polybius 3.25.6)