2 Samuel 20:8

When they were at the great stone which is in Gibeon, northwest of Jerusalem, Amasa went before them, coming towards them with the levy of troops which he had raised. And Joab's garment that he had put on was girded unto him, his military garment being held close to his body by the girdle, and upon it a girdle with a sword fastened upon his loins in the sheath thereof; and as he went forth, rather, the sheath slipped out, it fell out, that is, the sword fell to the ground. This apparent accident happened just before Amasa came up to Joab, and the fact that the latter picked up and held the sword in his left hand would arouse no suspicions.