The magic mirror is a folk motif with widespread - but particularly European - distribution. Stith Thompson is his monumental Motif-Index of Folk Literature (1956) classifies the motif as "D1323.1 Magic clairvoyant mirror." His distribution of sources demonstrate that this motif is not an oddity; rather is was common enough in folktales (lengthy, fictional stories told to entertain).

This does not address the question of the "origin of the motif". It merely provides an initial building block that could be used to begin the process of answering your question. It is next to impossible to answer questions about the origin of folk motifs, and anyone who attempts to answer your question with something definitive is likely speculating, and any such assertion cannot be proven.

This isn't always a satisfactory answer to these sorts of questions, but this is a typical limit that one encounters when attempting to pursue the origin of motifs. The Finnish Historic-Geographical Method attempts to zero in on where a folktale may have originated while also attempting to track the origin (time and place) of variations, but it is an elaborate process, and specific motifs - or even folktales - are typically waiting for this sort of study. Given that most modern folklorists do not subscribe to the Finnish method, the majority of academic research is directed to other subjects.