That's like being asked for a "source" for "The sky is blue," or "The grass is green," or "The sun sets in the west."

"Source"? England, France, Germany, Italy, Sweden, Denmark, Dearbornistan.

Walk down a dark street in a Muslim No-Go Zone at midnight. Go to a music festival wearing shorts. Be one of 10,000 victims of a Muslim rape gang, and try getting help from the police (who are terrified of being accused of "racism"). Be Kate Steinle. Be Jamiel Shaw.