To support what claim? That the Buddha took the ātman for granted (as axiomatic)?
Why do you think you need a sutta to tell you a point about grammar and semantics? I can clarify that no sutta is going to give you that kind of an assertion, because what you need is grammatical clarity, and the suttas are not there to replace grammar, or to fill the semantic void caused by an ignorance of Pali or Sanskrit grammar (and resulting confusions that arise by relying wholly or mainly on translations and/or mistranslations, or loss of meaning that happens as a matter of course when you switch languages).
You need to understand the difference between a paryudāsa-pratiṣedha and a prasajya-pratiṣedha in regard to the use of a nañ-samāsa. There, in grammar, and the construction of word-forms and compounds (as they apply to Sanskrit and Pali) you will find the answer to how you cannot semantically nullify a positive referent that you’ve compounded with ‘na’ to form an adjective.