Mahasatipatthana Sutta
ii. Iriyapatha Pabba (Section on Postures)

And again, bhikkus, a bhikkhu while walking note33 knows “I am walking”;note34 while standing, he knows, “I am standing”; while sitting, he knows, “I am sitting”; while lying down he knows, “I am lying down.”note35

To summarize, a bhikkhu should know whatever way his body is moving or placed.note36

Thus he dwells perceiving again and again the bodynote37 as just the body (not mine, not I, not self, but just a phenomenon) in himself; or he dwells perceiving again and again the body as just the body in others; or he dwells perceiving again and again the body as just the body in both himself and in others. He dwells perceiving again and again the cause and the actual appearing of the body or he dwells perceiving again and again the cause and the actual dissolution of the body; or he dwells perceiving again and again the actual appearing and dissolution of the body with their causes.note38 To summarize, he is firmly mindful of the fact that only the body exists (not a soul, a self or I). That mindfulness is just for gaining insight (vipassana) and mindfulness progressively. Being detached from craving and wrong views he dwells without clinging to anything in the world. Thus, bhikkhus, this is also a way in which a bhikkhu dwells perceiving again and again the body as just the body.