This is the second of the Great Antiphons/O Antiphons sung at the Magnificat at Vespers during the last days of Advent. This antiphon is traditionally used on December 18. (In the Paris Breviary it is used on December 16 and in medieval English usage it is on December 17.)
O Adonai, et Dux domus Israel, qui Moysi in igne flamme rubi apparuisti, et ei in Sina legem dedisti: veni ad redimendum nos in brachio extento.
Words: Anonymous, ca. 9th C.
Tune: “O Adonai” Gregorian Chant, Mode II, traditional.
This is the English translation of the text as it appears in Fr. Matthew Britt’s 1922 text of The Hymns of the Breviary and Missal:
O Adonai, and Leader of the House of Israel, who didst appear unto Moses in the burning bush, and gavest Him the Law on Sinai: come and redeem us by Thy outstretched arm.