The existence of allophones for phonemes like /ɹ/ in major dialects such as General Western English is a foundational fact of linguistics that requires no citation.
The claim that General Western English features specific allophones of the phoneme /ɹ/ is a standard textbook fact in phonetics and linguistics (e.g., retroflex vs. bunched realizations, syllabic variants, and context-dependent approximations). As an established observational fact of everyday linguistic description, it falls under common knowledge and does not require specific paper citations.