Somebody would have to prove in a Federal Court that ALL Mosques, or a majority of Mosques preach incitement to murder.
Or go after them one at a time.
Personally, I believe this is better solved by the independent actions of the citizenry in every city and town.
"No officer, I didn't see anything".
Mr. Trump never said that all mosques should be shut down anyways, just some of them.
No Establishment of Religion.