MN 10 Satipatthana Sutta: The Foundations of Mindfulness
The Blessed One said this: “This is the direct path for the purification of beings, for the overcoming of sorrow & lamentation, for the disappearance of pain & distress, for the attainment of the right method, & for the realization of Unbinding — in other words, the four frames of reference. Which four?
“There is the case where a monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. He remains focused on feelings… mind… mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.