When the Master used the formal speech, it was for the Songs, the Documents, and the rites.
论语 · 述而篇· our translation
Confucius switched to formal speech when reciting the Documents. 书 is the kind of text taken seriously.
孔子平时说方言，讲到《诗》《书》和行礼时才用雅言。「书」在这里指《尚书》——被郑重对待的那类文本。