The claim that the suffixes -ly and -y attach to different word classes to form adjectives is a matter of basic linguistic morphology and everyday grammar observation, meaning no external academic citation is strictly needed.
The claim outlines foundational linguistic facts regarding English derivational morphology (-y typically attaches to nouns to form adjectives like 'lucky', whereas -ly typically attaches to adjectives to form adverbs like 'quickly', though linguistic complexities exist). The retrieved papers touch on general morpheme and derivational category analysis, but do not directly or comprehensively test the comparative attachment classes of these specific suffixes. Consequently, the claim is best treated as common knowledge in linguistics while noting that the papers provide background context on word formation without settling the precise claim.