What is the earliest mention of the Zohar being explicitly cited in Jewish texts?
I've asked some friends but we were unable to determine where it was.
If you look at this article from this website: http://zomet.org.il/eng/?CategoryID=160&ArticleID=6835
It would seem according to Rav Ovadia Yosef that some parts of the Zohar labeled "midrash Rabbi Shimon Bar Yochai" were available and quoted as early as the time of the Aruch (Rav Nosson from Roma) who lived in the 11th century in Italy.
For a full anylisis of who quotes the Zohar, the earliest commentators etc. see https://en.m.wikipedia.org/wiki/Zohar