The battle in question was Napoleon's great victory at Austerlitz in which the French forces had managed to force a the Austro-Russian forces across the Satchan Pond and other waterways, as seen in this map of the retreat. Napoleon's 30th Bulletin claimed that during this phase of the battle:

The Emperor moved forward with 20 cannon. The corps was driven from position to position, and a dreadful spectacle ensued, such as we saw at Aboukir Bay: 20,000 men throwing themselves in the water and drowning in the lake!

This image of the French army forcing thousands of Russian troops to drown in the icy waters became a stock image in the Romantic imagery of the battle, such as Felician Myrbach's painting, this anonymous print which almost makes the demise of the Russians look like Pharaoh's at the Red Sea, or the 1960 film Austerliz.

The problem with this tale is that Napoleon's Bulletin is complete nonsense in this regard. The ponds of the area were not terribly deep and while some Russians did drown when the ice broke, these ponds' size could barely hold a few hundred corpses, let alone 20000. In 1811, Napoleon directed Suchet to drain the ponds and while the French found the remains of over a hundred horses, they found just three corpses. As the Baron de Comeau, a staff officer in the Grand Armee noted in his memoirs:

The Bulletin says that this pond absorbed thousands of them. I was near enough to see what occurred there. The Russian Army skirted the pond and put it between them and the cavalry that would have harassed them. While some parties might have gotten their feet wet, they did not drown. The few corpses of the men and horses that I saw there had been killed by gunfire; they were on the bank, and even nearer. I would say in fact that it was not 2000 men who perished, but fewer than 200 while without this obstacle the column would have perished.

Napoleon's embellished account in his Bulletin, which took some elements of actual events and magnified them to make the victory look even more of a reflection of his genius and foresight, has latched itself into popular memory, Napoleonic historians have long since demolished this particular tale of Austerlitz as fiction.

Source

Goetz, Robert. 1805: Austerlitz. London: Greenhill, 2005.