Witch? Is that the Witch-King or am I getting confused?
Nah, it's just a reference in the original appendices too. Basically, the three wizards and some of the elves were involved in driving the dark sorceror, the Necromancer out of his fortress in Dol Guldur. The Necromancer turns out to be Sauron.
It's only referenced in The Hobbit as business Gandalf had in the South, elaborated on briefly in LOTR and found in the appendix.