I've heard anecdotally that Bernhard Riemann was interested in applying his discovery/invention of Riemannian geometry to model gravitation about 50 years before Einstein managed to do so.

In this video, we hear Edward Witten say,

Riemann almost did it. Riemann had the conception that gravity was based on curved [...] space. He didn't conceive of curved spacetime, because special relativity hadn't happen yet, so time and space haven't been unified.

In this wikipedia article, we see written,

Similar to Newton, but mathematically in greater detail, Bernhard Riemann assumed in 1853 that the gravitational aether is an incompressible fluid and normal matter represents sinks in this aether. So if the aether is destroyed or absorbed proportionally to the masses within the bodies, a stream arises and carries all surrounding bodies into the direction of the central mass. Riemann speculated that the absorbed aether is transferred into another world or dimension.

These quotes sparked my interest in finding all the sources (preferably primary sources) where Riemann writes about gravity. This is for both where he considers curved space and the aether-based models.

Are there any sources to back up either one of the quotes?