Combine bananas and sour cream or buttermilk into a small bowl and mash together with a fork until well blended and creamy. The mixture can be used immediately or stored in a refrigerator overnight.
Place the banana mixture and the rest of the ingredients in the bread pan in the order listed.
Set the program to the Quick Bread/Cake cycle.
Set the crust setting to medium if offered.
The batter will be thick and smooth. After the first kneading cycle is finished, use a spatula to scrape any gathered flour mix from the corners and sides.
When the bread machine beeps that it is finished, you can check that the bread is done by verifying that the bread has shrunk slightly from the sides of the pan, and the top is firm to gentle pressure. A toothpick or metal skewer will come out clean when inserted into the center of the bread.
When done, immediately remove the pan from the machine.
Let the bread stand in the pan for 15 minutes before turning it out, right side up, to cool completely.
Wrap tightly in plastic wrap to store in the refrigerator.