(S.v.171)
388. Thus have I heard — At one time the Venerable Ānanda and the Venerable Bhadda were dwelling at Pāṭaliputta in the squirrels’ feeding ground. Then the Venerable Bhadda, having spent the day meditating in seclusion, approached the Venerable Ānanda. Having approached, he exchanged friendly greetings with the Venerable Ānanda. Having engaged in polite conversation, he sat down at one side. Sitting at one side, the Venerable Bhadda said to the Venerable Ānanda: “What is the root cause, friend Ānanda, what is the condition that the true Dhamma does not endure for a long time after the final passing away of the Tathāgata? What is the root cause, friend Ānanda, what is the condition that the true Dhamma endures for a long time after the final passing away of the Tathāgata?”
“Well said, friend Bhadda! Well said! You are fortunate, Bhadda, your question is skilful in seeking the the path. Thus you ask, friend Bhadda: ‘What is the root cause, friend Ānanda, what is the condition that the true Dhamma does not endure for a long time after the final passing away of the Tathāgata? What is the root cause, friend Ānanda, what is the condition that the true Dhamma endures for a long time after the final passing away of the Tathāgata?’”
“It is so, friend.”
“If the four foundations of mindfulness, friend, are not developed, not made much of, the true Dhamma does not last long after the final passing away of the Tathāgata. If the four foundations of mindfulness, friend, are developed, made much of, the true Dhamma lasts long after the final passing away of the Tathāgata.
“What four? Here, friend, a monk dwells contemplating the body in the body, ardent, clearly comprehending and mindful, having abandoned covetousness and sorrow concerning the world … contemplating feelings in feelings … thoughts in thoughts … mental objects in mental objects, ardent clearly comprehending and mindful, having abandoned covetousness and sorrow concerning the world. If the four foundations of mindfulness, friend, are not developed, not made much of, the true Dhamma does not last long after the final passing away of the Tathāgata. If the four foundations of mindfulness, friend, are developed, made much of, the true Dhamma lasts long after the final passing away of the Tathāgata.”