Hello, I was hoping someone could please help me identify a movie.

It was a horror movie from either the 70's or 80's, and all I recall is that a boy is in the kitchen chopping some vegetables or fruit and chops off his finger. His mother is swimming (I think) and comes in upset for having been disturbed, throws his finger into the sink and locks him in his room.

Thanks for the help!

(I have been trying to find the title for ages!)