The Kāyagatāsati Sutta (Skt. Kāyasmṛti; Mindfulness Immersed in the Body, MN 119) is a Pali Buddhist Sutta which outlines the development of mindfulness through contemplation of the body in order to reach Jhana.
Kayagatasati Sutta Wikipedia
The Kayagatasati sutta stresses the need for constant awareness of the body's position, "When walking, the monk discerns, 'I am walking.' When standing, he discerns, 'I am standing.' When sitting, he discerns, 'I am sitting.' When lying down, he discerns, 'I am lying down." The Sutta also outlines the practice of "reflections on repulsiveness of the body" (Patikulamanasikara). In this practice a meditator reflects on various parts of the body (nails, hair, bodily organs, fluids) and noting their impurity. The Sutta also recommends meditation on the impermanence of the body and death by contemplating human corpses in various states of decomposition. "Furthermore, as if he were to see a corpse cast away in a charnel ground — one day, two days, three days dead — bloated, livid, & festering, he applies it to this very body, 'This body, too: Such is its nature, such is its future, such its unavoidable fate'". The Sutta then explains the attainment of the first four rupa Jhanas, states of calm concentration reached through meditation.
Finally, the Sutta outlines the ten benefits of these practices which are as follows: