I don't think there is one single such piece, rather a continual roster of insinuation and inference establishing their current position.
However, here are some interesting antecedents:
Map depicting China's territory in South China Sea, by Ministry of the Interior, ROC, 1947
if it's ROC territory, then it must be PRC territory, by their logic.
also, Wikipedia notes that "In 1958, the People's Republic of China issued a declaration defining its territorial waters, which encompassed the Spratly Islands." but without a reference.