Is This Athar Regarding the Magians Being Considered Ahlul Kitab Authentic?

>They (also) differed on their position about Magians: Whether they are people of the book or not? As it was narrated from 'Ali ibn abi Talib may Allah be pleased with him that he said; They had a book which they used to study, and one day they "woke" up and their book was lifted (isra'), so that it was lifted from among them. > >*— Unknown Source* I apologize for not giving a source. I actually came across this athar from this site actually, but the answerer did not provide a source, nor a grading of this athar. As the title says, what is the authenticity of this athar? Likewise, if it's authentic, does it mean that the Magians were considered Ahlul Kitab, at least according to Ali that is?