I found that in some cultures or religions people think that the number 13 is an unlucky number, I'm wondering if this is accepted in Islam or not? and why?
Believing in superstitions is forbidden, everything comes from Allah, good and bad.
Omens (bad things,e.g number 13) is from the devil. He makes you think this is bad, so that your believe in god is less.
Follow this LINK it will tell you everything that you need, just know that these superstitions(omens) are all from the devil. If good or bad falls unto you its from Allah to test you.
Hope to have answered your question.