There are actually different translations of Bonhoeffer going around. The book I had read through is "Letters and Papers from Prison" - the enlarged edition by Dietrich Bonhoeffer (edited by Eberhard Bethge). However, in this book the word "stupidity" is translated rather as "folly". It's located in the first section titled "After Ten Years". First Touchstone Edition 1997

However, the edition that contains the word "stupidity" is actually the 2010 edition from Fortress Press.

Dietrich Bonhoeffer, Letters and Papers from Prison (Dietrich Bonhoeffer Works/English, vol. 8) Minneapolis, MN: Fortress Press 2010