Which planet's rings were discovered from Earth in 1977 by stellar occultation?

Uranus - and it was an accident. A team aboard a flying observatory was watching the planet pass in front of a star to measure its atmosphere, and the star blinked out five times before the planet arrived, then five times again afterwards, symmetrically. Something narrow and dark was orbiting it.

Occultation is one of the few ways to measure something too faint to see. You do not need to resolve the object at all; you need only precise timing of when a background star dims, and from the pattern you get sizes, positions and even atmospheric structure. The same technique found Chariklo's rings - a body only 250 km across, in 2013, which nobody expected to have rings at all - and is how Pluto's atmosphere was detected before any spacecraft went near it.

Uranus is peculiar in other ways. It rotates on its side, about 98 degrees over, probably from an enormous ancient collision, so each pole spends 42 years in continuous sunlight and 42 in darkness, and the rings and moons are tipped over with it. It was also the first planet ever discovered rather than simply known - Herschel found it in 1781 and tried to name it after King George III.