Abdul Al-Sayed:
It is likely that he swore by the Koran. Which Koran, we are not sure.
Thomas Jefferson had a copy of the Koran. So when Keith Ellison was sworn into Congress, he swore on this copy.
There is also the first Quran in the state of Michigan. The University of Michigan owns it. It is very old, and this may be the choice.
So I don't know, but I swear by the Koran.