This song is not in the Digital Tradition, and there is no entry in the Traditional Ballad Index. However, there is an entry on the song at Hymnary.org:

Some of the stanzas of this African American spiritual may date back to the eighteenth century. Other stanzas have been added by oral tradition. A look through modern hymnals will reveal an array of variations on the text. The most notable alteration in the Psalter Hymnal is the phrase "to the Lord of life" in place of the original "to the rising sun," in which "sun" was an ambiguous metaphor referring to God. The song's use at communion services probably dates from after the American Civil War. Miles Mark Fisher notes in Negro Slave Songs in the United States (1953),

[Originally the hymn] relates hardly at all to holy communion, which does not necessarily require early morning administration or a devotee who faces east. [This] it seems was a signal song of Virginia slaves during the eighteenth century who used it and similar ones to convene their secret meetings.

The text discerns participation in the Lord's Supper as a humble act in which we not only eat the bread (st. 1) and drink the wine (st. 2) but also praise our God (st. 3) "on our knees." The refrain ends with a prayer for mercy, an African American kyrie (see PHH 258) that reminds us of the tax collector's prayer in Luke 18:13.

Liturgical Use:
Lord's Supper–during preparation for the sacrament or during distribution of the bread and wine.
--Psalter Hymnal Handbook, 1988

The tune BREAK BREAD TOGETHER, like the text, has been subject to variation. It became widely known after publication in The Second Book of Negro Spirituals (1926), compiled by the brothers James Weldon Johnson and Rosamond Johnson. The tune gained further popularity through a variety of choral arrangements; it can be found in many hymnals dating after 1955, when it was published in the American Presbyterian/Reformed Hymnbook. Dale Grotenhuis (PHH 4) harmonized the tune in 1984 for the Psalter Hymnal.

Arranged without the call-and-response pattern that often characterizes African American spirituals, BREAK BREAD TOGETHER in the Psalter Hymnal takes the shape of a regular hymn, with part singing on the stanzas and refrain. If you like, however, sing stanzas 1 and 2 in unison and the refrain and stanza 3 in parts–a higher melody line for stanza 3 is published in The Hymnal 1982 (1985), a revision of the American Protestant Episcopal Hymnal 1940. In addition, try singing the entire song without accompaniment.

--Psalter Hymnal Handbook, 1987

Full text:

1 Let us break bread together on our knees;
let us break bread together on our knees;

Refrain:
When I fall on my knees,
with my face to the Lord of life (rising sun),
O Lord, have mercy on me.

2 Let us drink wine together on our knees;
let us drink wine together on our knees. [Refrain]

3 Let us praise God together on our knees;
let us praise God together on our knees. [Refrain]

Source: Lift Up Your Hearts: psalms, hymns, and spiritual songs #837