Hi Lal,
The source is The Truth Taught by All the Buddhas – Bhikkhu Revata
Not sure whether you guys can view it via the link above. A full version copy can be purchased at $0.00 though.
Pages 56, 57, 81, 118 are some of the references that might be of interest to fellow readers.
From page 118:
“The Buddha instructs us next to stand somewhere, develop concentration, and then start walking with close eyes. Wherever one goes, there is sound. It impinges on one’s ear-door and mind-door. Ear-door mental process and mind-door mental process arise. One’s eyes are closed, but one sees one’s whole body as just very small sub-atomic particles arising and perishing very rapidly. When the sound impinges on the ear and mind-doors, one sees the mental processes arising and perishing very rapidly. Then one does not see one’s body; one sees only ultimate mentality and materiality arising and perishing very rapidly all the time. If one walks for an hour, one sees only this. If one pays attention to external persons, one sees nothing other than this. The Buddha instructs us to reflect at that time that there is no man, there is no woman, there is no deva, there is no brahmā, there is no permanent en tity, there is no permanent soul, there is only ultimate mentality and materiality. One understands what the Buddha meant. One agrees with the Buddha that no such things exist. There is only ultimate mentality and materiality.”