I claim the ornamental design for a power source, as shown and described.
FIG. 1 is a front perspective view of a power source.
FIG. 2 is a front view of the power source shown in FIG. 1.
FIG. 3 is a rear view of the power source shown in FIG. 1.
FIG. 4 is a first side view of the power source shown in FIG. 1.
FIG. 5 is a second side view of the power source shown in FIG. 1.
FIG. 6 is a top view of the power source shown in FIG. 1; and,
FIG. 7 is a bottom view of the power source shown in FIG. 1.
The broken lines having equal segments represent portions of the article that form no part of the claimed design. The broken lines having unequal length segments illustrate the boundary of the claimed design and form no part of the claimed design.