Sorry, a key bit missing from the question. It should have been where does it state that Form SA 106 (Foreign) does not need to be filled in by non-UK residents? I know that non-UK source income is not taxable - the question is about filing obligations.
Where does it say in returns, notes, guidance, legislation or similar that SA does not need to be filled in by non-UK residents? I have seen many HMRC and other statements that this is the case but cannot find the source.