2 Samuel 20:8

When they [were] at the great stone which [is] in Gibeon, Amasa went before them. And Joab's garment that he had put on was girded unto him, and upon it a girdle [with] a sword fastened upon his loins in the sheath thereof; and as he went forth it fell out.

Ver. 8. And as he went forth, it fell out.] He had taught it, as it were, at a certain motion or posture of his body, to fall out of the scabbard, that taking it up in his hand again, he might inter salutandum smite Amasa, suspecting no harm.