My Chess Life

MATE IN ONE

Here is an interesting puzzle I found in a book (I'll tell you later with the solution).

The question:

Peter was watching his two brothers playing a game of chess. At one point of the game, Peter sketched out exactly what he saw on the board, and his diagram is shown here. The interesting thing is that it was his eldest brother David's move next (he was playing with the white pieces), and he achieved checkmate that move.

What was David's move?