FIG. 1 is a perspective view of a knife sheath showing my new design; FIG. 2 is a front elevational view, FIG. 3 is a rear elevational view, FIG. 4 is a left side elevational view, FIG. 5 is a right side elevational view, FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof. The broken line showing is for illustrative purposes only and forms no part of the claimed design.