The exact quote is from the Weimar edition of Luther’s works 10[2], 212 translated in Robert C. Croken’s
Luther’s First Front: The Eucharist as Sacrifice:
It is very clear that the Mass is not a matter of our work or word, but of Christ alone giving us both the word of promise and the sign of bread and wine, and that its celebration consists not in offering or sacrificing, but only in receiving and benefiting.
This was written in response to Henry VIII’s attack on Luther, for which Henry was given the title “Defender of the Faith” by the Pope.
The Eucharist in the West, edited by Edward J. Kilmartin, SJ, and Robert J. Daly, SJ, and published by Liturgical Press in 2004, states:
Luther was convinced that the Catholic doctrine of the sacrifice of the Mass describes only a human work, and that it makes of this work a means of grace. . . . This explanation of Biel’s provided strong support for Luther’s conviction that Catholic theology erroneously attributes to the Mass a sacrificial activity that competes with the sacrifice of the cross. For, in this theory, the victim of the cross and the Mass is identical, but* not the sacrifice*. The eucharistic sacrifice is offered by the Church acting as principalis offerens. (p. 172).