This is what I heard— At one time the Buddha was staying in the land of the Kurus, where they have a city called Kammāsadhamma. There the Buddha addressed the monastics: “Monastics!” “Venerable sir”, they replied. The Buddha said this:

“Monastics, this is the path where all things come together as one, to purify sentient beings, to make an end of pain and sadness, to get past sorrow and lamentation, to reach the way, to witness Nibbāna; that is, the four kinds of mindfulness meditation.

What four? Here, a monastic meditates by observing an aspect of the body, keen, aware, and mindful, rid of desire and aversion for the world. They meditate by observing an aspect of feelings, keen, aware, and mindful, rid of desire and aversion for the world. They meditate by observing an aspect of the mind, keen, aware, and mindful, rid of desire and aversion for the world. They meditate by observing an aspect of principles, keen, aware, and mindful, rid of desire and aversion for the world.