The song you want is "Down In The River To Pray." It was performed by Alison Krauss. It is on a compilation CD distributed with an issue of The Oxford American magazine, and also on the sound track album for "O Brother, Where Art Thou?" (which you can also find here). It is described as a "traditional spiritual" but I have been unable to find the lyrics.

By the way, looking at the contents of these CD's, they both sound like great albums!