There is no source because they made it up. The closest thing I could find is this https://news.yahoo.com/wagner-mercenaries-involved-training-hamas-145700770.html Which states wagner group may have trained Hamas fighters according to this report https://sprotyv.mod.gov.ua/instruktory-pvk-vagner-gotuvaly-hamas-do-ataky-na-izrayil/
Here's a great video Shaun made in response to the CGP one.