I'm wracking my brain to remember where I read about the vikings in the Ottoman Empire, I know that I read it before the internet was a significant thing. It might have been from Ottoman Centuries, which I remember being a pretty good book on the Ottoman Empire but I'm not sure.
In Russia, it is held to be common knowledge that there has been active connection between the Scandinavians and the Byzanthium. Those were two main trade and culture partners for the Rus' of the 10th century. A prominent theory even states that a Norseman came to establish rule in Rus' at the call of the Rus' people. (Alternative theories suggest that it was a Rus' man, or that there was no Rurik at all — there's always one such theory somewhere) Seems like they went a bit further than Byzanthium, or that Muslims were in contact with the Varangians in Constantinople. Which the Eastern Roman Empire was Christian, I wonder if that meant that Muslims weren't allowed in.