hmm, well lots of sites.. but can't find much yet specific to this prediciton/prophesy.. would help if I could speak Arabic of course... which I sadly don't.. anyway.. here's a bit from Wikipedia on this man, who seems to have been a very important figure in early Islam, and also the roots of Islamic Mysticism, Sufism and is reffered to by Kahlil Gibran ('The Prophet' author)... is all interesting! (sorry if not strictly Avalon material !)
http://en.wikipedia.org/wiki/Ali_ibn_Abi_Talib