Saṃyukta Āgama 610 Correct 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 said to the bhikṣus, “I will speak of cultivating the Four Bases of Mindfulness. Listen carefully and consider well.
“How are the Four Bases of Mindfulness cultivated? That is to say,  dwelling in mindfulness of the body, observing the inner body: ardent, aware and mindful, setting aside worldly sorrow and distress; and of the outer body; and of dwelling observing both inner and outer body: ardent, mindful and aware, setting aside worldly sorrow and distress. It is also such as this  for sensations;  for the mind;  and for dharmas, dwelling in the mindfulness of observing inner dharmas, outer dharmas, and both inner and outer dharmas: ardent, mindful and aware, setting aside worldly sorrow and distress. This is what is meant by a bhikṣu cultivating the Four Bases of Mindfulness.”
After the Buddha had spoken this sūtra, the bhikṣus heard what the Buddha had said, and joyfully practiced in accordance.