Per the Wikipedia article:
At the end of the twelfth century al-Malek al-Aziz Othman ben Yusuf, Saladin's son and heir, attempted to demolish the pyramids starting with Menkaure's pyramid. The workmen who Al-Aziz had recruited to demolish the pyramid found it almost as expensive to destroy as to build. They stayed at their job for eight months. They were not able to remove more than one or two stones each day at a cost of tiring themselves out utterly. Some used wedges and levers to move the stones while others used ropes to pull them down. When it fell it would bury itself in the sand requiring extraordinary efforts to free it. Wedges were used to split the stones into several pieces and a cart was used to carry it to the foot of the escarpment, where it was left. Far from accomplishing what they intended to do they merely spoiled the pyramid by leaving a large vertical gash in its north face
Sources for the Wikipedia section are:
Stewert, Desmond and editors of the Newsweek Book Division "The Pyramids and Sphinx" 1971 p. 101
Lehner, Mark The Complete Pyramids, London: Thames and Hudson (1997)p.41 ISBN 0-500-05084-8
The page on ben Yusuf states
During his reign, he tried to demolish the Great Pyramids of Giza, Egypt, but had to give up because the task was too big. However, he did succeed in damaging Menkaure's Pyramid.
Wikipedia's source for that statement is
- Way, The. "Why Western Art Is Unique, and Why Muslim Immigration Threatens It | The Brussels Journal." The Brussels Journal | The Voice of Conservatism in Europe. Web. 29 June 2010. http://www.brusselsjournal.com/node/2128.
This source states it occured in 1193. the source for that source is http://egyptologist.org/discus/messages/8/4300.html, which goes into great detail on the damage. An excerpt:
When king Al-Aziz Othman, son of [Saladdin] succeeded his father, he let himself be persuaded by some people from his Court, who were devoid of good sense, to demolish the pyramids. One started with the red pyramid, which is the third of the great pyramids, and the smallest. So the sultan sent sappers, miners and quarrymen, lead by some of the main officers and the first emirs of his Court and ordered them to destroy it.... This happened in the year 593 (i.e. 1196 A.D.)." (transl. SACY, Description de l'Egypte IX, 468).
He sources Description de l'Egypte IX, 468, which is in French. I can't seem to find any sources for the book (probably because I don't speak French).