georone 说:
flags of Muslims are very simple.
They are green, black or crescent
or letters in Arabic like this:
That's actually not true. Today they are green, black or with crescent, but back then it was not like this. As far as I know, we have little clue of how their banners were.
Arabic verses from the Qur'an, probably, although we do know that yellow (golden) was the colour of the Ayyubids, and that they had an eagle as their ensign. Also, that the Fatimids used plain green flags (no crescents anywhere).