The Cause and Reason Why the True Dhamma does not endure Long after a Tathagata has attained final Nibbāna
THUS HAVE I HEARD. On one occasion the Blessed One was dwelling at Savatthi, in Jeta’s Grove, Anathapiṇḍika’s Park. Then a certain brahmin approached the Blessed One and exchanged greetings with him. When they had concluded their greetings and cordial talk, he sat down to one side and said to the Blessed One:
“Master Gotama, what is the cause and reason why the true Dhamma does not endure long after a Tathagata has attained final Nibbāna? And what is the cause and reason why the true Dhamma endures long after a Tathagata has attained final Nibbāna?”
“It is, brahmin, because the four establishments of mindfulness are not developed and cultivated that the true Dhamma does not endure long after a Tathagata has attained final Nibbāna. And it is because the four establishments of mindfulness are developed and cultivated that the true Dhamma endures long after a Tathagata has attained final Nibbāna.
What four?
Here, brahmin , a bhikkhu dwells contemplating the body in the body, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.
dwells contemplating feelings in feelings, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.
dwells contemplating mind in mind ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.
dwells contemplating phenomena in phenomena, ardent, clearly comprehending, mindful, having removed covetousness and displeasure in regard to the world.
It is because these four establishments of mindfulness are not developed and cultivated that the true Dhamma does not endure long after a Tathagata has attained final Nibbāna. And it is because these four establishments of mindfulness are developed and cultivated that the true Dhamma endures long after a Tathagata has attained final Nibbāna.”
When this was said, that brahmin said to the Blessed One: ‘Magnificent, Master Gotama!… From today let Master Gotama remember me as a lay follower who has gone for refuge for life.”
Source: https://suttacentral.net/sn47.25 Aññatarabrāhmaṇa Sutta SN 47.25 SN v 174