Forgive me for the vague question. Several years ago, I read about a theorem that (if I remember well) proved what is written in the title. I guess the theorem is from Wiener and it seemed that this theorem was present in his book "The Fourier integral and certain of its applications". But I can't find it anywhere. I could be mistaken in my attribution though.
I've been Googling for several days and I couldn't find it. Perhaps I don't remember it too well, perhaps me having to translate from my native language has caused some trouble, etc.