Name of a peter sellers movie that had line my god but your lovely?

The movie you're thinking of is "Being There" (1979).

In the movie, Peter Sellers plays Chance the gardener, a simple-minded man who is mistaken for a brilliant philosopher. The line "My God, but you're lovely" is spoken by Shirley (played by Shirley MacLaine) when she first meets Chance and is captivated by his innocent charm.