大毘盧遮那經供養次第法疏 1.30

1.30

「我淨此身離諸垢」以下，第四施身方便真言門，亦有誦真言。真身以外無別身，三世如來亦同體，眾生妄執有別身，此真言門能令解我身佛身等無別，故言施身。

From "I purify this body, free from all defilements" onward is the fourth, the mantra gate of the upāya of offering one's body; it also has the recitation of a mantra. Outside of the true body there is no other body, and the Tathāgatas of the three times are of the same essence. Sentient beings falsely cling to having a separate body; this mantra gate enables them to understand that one's own body and the Buddha's body are identical and without distinction, and therefore it is called offering one's body.