At Savatthi.

“Bhikkhus, dwell with your minds well established in the four establishments of mindfulness. Do not let the Deathless be lost on you.

“In what four?

Here, bhikkhus, a bhikkhu dwells contemplating the body in the body, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.

dwells contemplating feelings in feelings, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.

dwells contemplating mind in mind, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.

dwells contemplating phenomena in phenomena, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.

Dwell, bhikkhus, with your minds well established in these four establishments of mindfulness. Do not let the Deathless be lost on you.”

Source:

Amatasutta SN 47.41 https://suttacentral.net/sn47.41