,,I was a good monk." - Luther

I have been trying to find the source of the following quote all day. Does anyone have a reference in Luther's Works where he says this?

,,I was a good monk, and I kept the rule of my order so strictly that I may say that if ever a monk got to heaven by his monkery it was I. All my brothers in the monastery who knew me will bear me out. If I had kept on any longer, I should have killed myself with vigils, prayers, reading, and other work."