Hi Marc
more talks can be found from Ajahn Sumedho in the link I gave above regarding the books, Abhiyagiri may send people to either Amaravati or Forest sangha publications though as Ajahn Sumedho was the abbot there.
but, the closest I know regarding the Buddha teaching this is passages on clear knowing, and "in the heard there is only the heard" I doubt it was actually taught directly by the Buddha in the way Ajahn Sumedho does, although just to note (not thinking you or anyone is suggesting this) that doesn't mean that it isn't useful as a tool particularly for people with tinitus.
