Saṃyukta Āgama 606
Smṛtyupasthāna Bases of Mindfulness
Thus have I heard. At one time the Buddha was dwelling in Śrāvastī, in the Jeta Grove, in Anāthapiṇḍada’s Park.
At that time, the Bhagavān told the bhikṣus, “There are four bases of mindfulness. What are these four? That is to say,  the base of mindfulness of the body, observing the body;  of sensation;  of the mind; and  the base of mindfulness of dharmas, observing dharmas. Thusly, bhikṣus, these four bases of mindfulness are cultivated to fulfillment. With ardency, mindfulness, and awareness, they should be learned.”
After the Buddha had spoken this sūtra, then the bhikṣus heard what the Buddha had said, and joyfully practiced in accordance.