Given $1\leq p\leq\infty$, prove that if it exists a constant C s.t. $$\|\hat{f}\|_{p'}\leq C \|f\|_p$$here $1/p+1/p'=1;$ then $1\leq p\leq 2$
I know that if $1\leq p\leq 2$, then it exists a constant C s.t. $$\|\hat{f}\|_{p'}\leq C \|f\|_p$$ and I have no idea how to prove this result.
Any idea will be helpful. Thanks.