morphism of constant sheaves

Question

How to show that if $\phi$ is a morphism of sheaves from $K_X$ ro $K_X$, whare $K_X$ is the constant sheaf on $X$, (where $X$ is connected, and locally commected,) then there exista $a$ in $K$ such that $\phi(U)=$'multiplication by a' for all open $U$ in $X$. $K$ is any commutative ring with unity. please help. Thanks in advance.