The following is quoted from John W. Work, 1915, Folk Song of the American Negro, reprint 1969, Negro Universities Press; pp. 78-80. It expands on a story quoted by Azizi:

""Swing Low, Sweet Chariot" and "Before I'd-be a Slave," upon first hand authority, may be called the "Twins," for they burst from the same soul of anguish. These songs were born from the same heart at the same time and under the same condition.

"Before I'd be a slave,
I'd be buried in my grave,
And a-don't let it catch you with your work undone.

A master of a Tennessee plantation had sold a mother from her babe, and the day for the separation was fast approaching when the mother was to be taken "down South." Now, the condition of the slave in Tennessee was better than in any other state, with the possible exception of Virginia. To be sold "South" was, to the slave, to make the journey from which no traveler ever returned. So it was not strange that the mother would sooner take her life and that of her babe, then to go down into Mississippi, which, to her, was goin to her grave. Bent upon throwing herself and her child over the steep banks of the Cumberland River, she was stumbling along the dusty road, her infant clasped close to her breast, muttering in frenzy her dire determination. "Before I'd be a slave, I'd be buried in my grave!" An old "mammy,' seeing the terrib expression on her face, and hearing these words, read her intentions. In love she laid her dear old hand upon the shoulder of the distressed mother and said, "Don't you do it, honey; wait, let the chariot of de Lord swing low, and let me take one of de Lord's scrolls an' read it to you." Then, making a motion and reaching for something and unrolling it, she read, "God's got a great work for dis baby to do; she's goin' to stand befo' kings and queens. Don't you do it, honey."
The mother was so impressed with the words......she allowed herself to be taken off.... leaving her baby behind. The prophecy of the old "mammy" was literally fulfilled. After the war, the baby girl entered Fisk University and was a member of the original Fisk Jubilee Singers...... When the tour of the singers was ended, this girl set out to find her mother........found her and brought her into a beautiful home, where she lived in love and comfort until the summer of 1912, when the "Sweet Chariout Swung Low" and bore her home. ............That baby girl was Ella Sheppard, who afterwards became pianist of the Original Jubilee Singers. At the annual meeting of the American Missionary Association........he requested the Fisk Quartette to sing for him "Swing Low, Sweet Chariot"..... A few days later, the chariot swung low and bore the General home.
"Most done toilin' here;
Um! Most done toilin' here!

"This song was born since freedom and is one of the very few real folk songs that were produced by freedom. It is really a new song of Virginia."

Work goes on to give a story about "Great Camp Meeting."

The book of Maud Cuney-Hare has tha same story about the separation of the mother and babe, but does not mention the name of the girl.

Azizi, I think, correctly refers to the story as "legendary;" it is typical of abolitionist stories, and the publicity associated with the Fisk Jubilee Singers.