# For references on termwise differentiation of Fourier series?

I have seen a result on termwise differentiation of Fourier series:

If$$f$$ is periodic with period $$2l$$, continuous on $$\mathbb{R}$$ and $$f'$$ is piecewise continuous on $$[−l,l]$$, then the Fourier Series of $$f$$ can be differentiated termwise.

Above result is migirated from Doubt Regarding Termwise differentiation of Fourier Series . It is really useful to my current research. Could anyone help with reference on above result?