Was that the chap who scolded Muhammad for being lax in his observance of the religion?
I don't think so, the "mad caliph" (Al-Hakim bi-Amr Allah) was just a little mad... http://en.wikipedia.org/wiki/Mad_Caliph Here's his wikipedia entry.
He was not very nice towards christians, he for an example made them wear black hats and they had to wear one meter-long crosses on their backs.