Sarah H. Bradford's Harriet: The Moses of Her People (Geo. R. Lockwood & Son, 1886) seems to be one of the main sources for the story of Tubman as Moses in this song. The following is from the relevant pages (pp. 37-38).

Up and down the road she passes to see if the coast is clear, and then to make them certain that it is their leader who is coming, she breaks out into the plaintive strains of the song, forbidden to her people at the South, but which she and her followers delight to sing together:


                           Oh go down, Moses,
                           Way down into Egypt's land,
                           Tell old Pharaoh,
                           Let my people go.

                           Oh Pharaoh said he would go cross,
                           Let my people go,
                           And don't get lost in de wilderness,
                           Let my people go.

                           Oh go down, Moses,
                           Way down into Egypt's land,
                           Tell old Pharaoh,
                           Let my people go.

                           You may hinder me here, but you can't up dere,
                           Let my people go,
                           He sits in de Hebben and answers prayer,
                           Let my people go!

                           Oh go down, Moses,
                           Way down into Egypt's land,
                           Tell old Pharaoh,
                           Let my people go.

       And then she enters the recesses of the wood, carrying hope and comfort to the anxious watchers there. One by one they steal out from their hiding places, and are fed and strengthened for another night's journey.

~Masato