The 'offer' being 'do as I say or I'll kill you'.

Line from The Godfather book and film, both written by Mario Puzo. It is used in all of the films of the Godfather trilogy but varies slightly throughout and isn't always easy to hear through all that cotton wool. In the first occurrence in the first film it is 'I'll make him an offer he don't refuse':

Clemenza: His family's out of the house. Fanucci's alone in the cafe.

Tessio: Vito, here's my fifty dollars - Buona fortuna (good luck).

Clemenza: Hey Vito, are you sure he's going to go for it?

Vito: I make an offer he don't refuse. Don't worry.