“Monks, this is the one and only path for the purification of beings, for the overcoming of sorrow and lamentation, for the disappearance of pain and grief, for the attainment of the true way, for the realization of Nibbāna. In other words, the four establishments of mindfulness. What are the four? Here, monks, a monk remains contemplating the body as body, dedicated, fully aware, and mindful, having abandoned greed and grief for the world. He remains contemplating feelings as feelings, dedicated, fully aware, and mindful, having put away greed and grief for the world. He remains contemplating mind as mind, dedicated, fully aware, and mindful, having put away greed and grief for the world.“He remains contemplating phenomena as phenomena, dedicated, fully aware, and mindful, having put away greed and grief for the world.” - Buddha -
This is a detailed teaching on the Foundations of Mindfulness Sutta, Maha Satipatthana Sutta, found in the Digha Nikaya. It includes numerous other suttas found in the Sutta Pitaka to make these teachings clear and complete.