Either soak the beans overnight in a pot of water or quick soak the beans by placing them in a large pot and cover the beans with water by 2 inches. Bring the water to a boil and cook for 10 minutes. Remove the pot from the stove and place a lid on the pot. Let beans sit for 1 hour.
Rinse and drain the beans and set aside.
In a large pot, heat the oil over a medium high heat. Add the onions, celery and bell pepper to the pot and cook, stirring frequently for 5 minutes. Add the garlic and cook for 3 minutes more.
Add the chopped sausage to the pot and cook along with the vegetables for 4 minutes, stirring occasionally.
Add the beans and 8 cups of water to the pot. Add in the bay leaves, salt, black pepper, cayenne pepper and thyme. Stir well.
Bring the beans to a boil, then reduce heat to low and simmer uncovered about 2 hours, stirring occasionally.
After the 2 hours and when the beans are tender, stir in the butter and add salt and pepper to taste.
If the beans become too thick and dry, add a little water.
Using the back of a heavy spoon, mash a few of the beans against the side of the pot. Remove from heat, take out the bay leaves and serve over rice.