Saṃyukta Āgama 615
Thus have I heard. At one time the Buddha was abiding in Śrāvastī, in the Jeta Grove, in Anāthapiṇḍada’s Park. At that time, in the early morning, Venerable Ānanda arranged his robes, picked up his bowl, and entered the city of Śrāvastī to beg for food. Along the road, he thought, “I should first go to the residence of the bhikṣuṇīs.” He then promptly went to the residence of the bhikṣuṇīs.
The bhikṣuṇīs saw Venerable Ānanda coming from afar. They quickly arranged a seat for him, and invited him to be seated. Then the bhikṣuṇīs bowed at the feet of Venerable Ānanda and withdrew to one side. They addressed Venerable Ānanda, saying, “We bhikṣuṇīs cultivate the Four Bases of Mindfulness, dwelling steadfastly, with self-knowledge of before and after, rising and falling.”
Venerable Ānanda said to the bhikṣuṇīs, “Excellent, excellent, sisters! They should be trained as you have said. One who cultivates the Four Bases of Mindfulness, dwelling steadfastly, should thusly know of before and after, rising and falling.” Then Venerable Ānanda spoke of various dharmas to the bhikṣuṇīs. After speaking of various dharmas, he arose from his seat and departed.
At that time, Venerable Ānanda begged for food in Śrāvastī and returned. He put away his bowl, washed his feet, and went to the place of the Bhagavān. He bowed his head at the Buddha’s feet and withdrew to one side. He spoke to the Bhagavān about what he had told the bhikṣuṇīs.
The Buddha said to Ānanda, “Excellent, excellent! The Four Bases of Mindfulness should be cultivated thusly: dwelling steadfastly, knowing before and after, rising and falling. Why is this so? The mind seeks the external, and as a result seeking arises in the mind, and the mind is scattered, unable to understand how things truly are.
“Suppose a bhikṣu  abides in mindfulness of the body, observing the body. Having abided in mindfulness of the body, observing the body, suppose his body is sluggish, or his mind and dharmas are remiss. That bhikṣu should give rise to pure faith, grasping hold of this pure manifestation. Giving rise to a mind of pure faith, recollecting this manifestation of purity, his mind becomes gladdened. Gladdened, there is the arising of joy. The mind being joyful, the body becomes pliant and calm. The body being pliant and calm, the body feels bliss. The body feeling bliss, the mind becomes concentrated.
“Concentrated, the noble disciple should train, ‘According to this principle, the mind scattered outside is controlled and pacified, not giving rise to vitarka and vicāra. Without vitarka and vicāra, one abandons reflections and abides in bliss. Abiding in bliss, one understands how things truly are.’  For sensations,  for the mind,  and for dharmas, it is also said thusly.”