How does the saying go about people who accuse their partner of cheating?

I cant quite remember the saying but its basically saying when for ex you bf is quick to accuse you of cheating he himself is probably the one doing the cheating.......How does the saying go about people who accuse their partner of cheating?
yeh thats right but also they might accuse you because there last ex cheated on him or he might be doin that to get out of the relationship or scared he might do it to you so he accuses you so talk him dont get angry but ask what going on or maybe not cause guys dont like ';the talk';