The claim that do-support applies in English wh-questions when the subject is not the questioned element describes a standard, descriptive rule of English grammar (e.g., 'What did John see?' versus 'Who saw Mary?'), so no citation is needed.
The claim states a basic descriptive rule of English syntax regarding subject/non-subject asymmetries in wh-questions and do-support. This is a foundational grammatical fact taught in introductory linguistics and grammar, representing common knowledge in the field that does not require empirical paper receipts.