Do you have a book or other reference for that?
I could find only one direct reference that Lewis made about Islam, which he described as "a Christian heresy."
http://www.kirjasto.sci.fi/cslewis.htm
Some say Lewis references Islam in 'The Chronicles of Narnia' )(the "Calormenes":)
He talks about religious principles in 'The Abolition Of Man'
I did find this info at Town Hall, by By Mona Charen
Friday, December 14, 2001