金剛經纂要刊定記 6.99

6.99

谷中下。喻明也。意云：以有外聲遂有響答，谷中實無作響之者。說法亦爾，法身實無能說之者，以機感故，遂見如來有所說法。又谷雖應聲而無應聲之念，佛雖說法而無說法之心。據此却由無念故，方能說法。是故遮云勿謂等也。

From "Within a valley" downwards uses a metaphor for clarification. The meaning is: because there is an external sound, an echo answers, but within the valley there is actually no maker of the echo. Preaching the Dharma is also like this: in the Dharmakāya there is actually no one capable of preaching, but because of the resonance of capacities [of beings], one subsequently sees that the Tathāgata preaches the Dharma. Furthermore, although the valley responds to the sound, it has no thought of responding to the sound; although the Buddha preaches the Dharma, he has no mind to preach the Dharma. Based on this, it is precisely due to having no thought that he can preach the Dharma. Therefore, it refutes [it] by saying "Do not say," etc.