Luther on the Protestant work ethic

It is often claimed that Luther defended the value of all vocation by saying:

The maid who sweeps her kitchen is doing the will of God just as much as the monk who prays -- not because she may sing a Christian hymn as she sweeps but because God loves clean floors. The Christian shoemaker does his Christian duty not by putting little crosses on the shoes, but by making good shoes, because God is interested in good craftsmanship.

I've tried tracing this quotation in Luther to no avail. It's all over the internet mind you as one of Luther's sayings but no one ever sources where in Luther it comes from. Anyway I midst my searching I did come across something good that Luther did actually say regarding vocation:

The prince should think: Christ has served me and made everything to follow him; therefore, I should also serve my neighbour, protect him and everything that belongs to him. That is why God has given me this office, and I have it that I might serve him. That would be a good prince and ruler. When a prince sees his neighbour oppressed, he should think: That concerns me! I must protect and shield my neighbour. . . . The same is true for shoemaker, tailor, scribe, or reader. If he is a Christian tailor, he will say: I make these clothes because God has bidden me do so, so that I can earn a living, so that I can help and serve my neighbour. When a Christian does not serve the other, God is not present; that is not Christian living. 

– Martin Luther, “Sermon in the Castle Church at Weimar” (25 October 1522, Saturday after the Eighteenth Sunday after Trinity)

In other words, Christians have such great liberty (and should be the most productive) because whatever lawful work they embark on - beer making, child rearing, road sweeping, chimney cleaning, love making, all that (and more!) can be considered the Lord's work. Praise be to our God and His Christ, for designing His world and our work in it to carry with such immense value. As Herbert put it: the world is charged with the grandeur of God. Indeed. And our labours therefore last into eternity.