i could not Edit my answer so i have posted new answer
1)let $k<1$ and $f(x)=a^{x}$ in which $a>1$ so
$f'(x)=a^{x}\log a$ and $f(x+k)=a^{x+k}$ then $k=\log \log a/\log a$ this is hold for $k<1$
2)let $k\ge 1$ and $f(x)=g(x)a^{-x}$ in which, $1<a<1.76$, and $g(x)=-1$, if, $x>1$, $g(x)=1$
if $0<x<1$ ,otherwise $g(x)=0$, so for $0<x<1$,
$f'(x)=-a^{-x}\log a$ and $f(x+k)=-a^{-x-k}$ then $k=-\log \log a/\log a$ this is hold for $k\ge 1$