Saṃyutta Nikāya 47
Connected Discourses on the Establishments of Mindfulness
“Bhikkhus, there are these three feelings. What three? Pleasant feeling, painful feeling, neither-painful-nor-pleasant feeling. These are the three feelings. The four establishments of mindfulness are to be developed for the full understanding of these three feelings.
“What four? Here, bhikkhus, a bhikkhu dwells contemplating the body in the body … feelings in feelings … mind in mind … phenomena in phenomena, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.