This actually gets asked from time to time.

For the most recent case and a good answer to it, see LegalAction's answer to this thread.

The general gist is that it's not really a claim that can be backed up by evidence, and as such isn't something you should be taking seriously as an historical fact.

Otherwise here are some of the other previous times the question or a similar one has been addressed:

• Is There Any Evidence That Moors Reached The Americas Before Columbus?

• How long did it take for knowledge of the "discovery" of America to make it to the Muslim world? Did any Muslim leaders send subsequent expeditions to the New World?

• Can anybody elaborate on the Theory that Africans navigated across the Atlantic long before Columbus?

• Is there any truth to claims that Africans arrived in the New World before Columbus' voyages?

• I've heard rumours since my grade 11 history class that the Egyptians crossed the Atlantic and visited North America long before the rise of the Roman civilization. Does anyone have sources confirming this, or is it just a rumour?