Saṃyuktāgama 114. [Discourse on Understanding Dukkha]
Thus have I heard. At one time the Buddha was staying at Mount Makula with an attendant monk by the name of Rādha.
Then a group of heterodox wanderers approached the venerable Rādha. Having exchanged polite greetings, they withdrew to sit at one side. They asked Rādha:
“For what sake have you gone forth to cultivate the holy life under the recluse Gotama?”
Rādha replied: “I have gone forth to cultivate the holy life under the Blessed One for the sake of understanding dukkha.”
Then the heterodox [wanderers], hearing what Rādha had said, were not pleased in their minds. They rose from their seats, blamed him, and left.
At that time Rādha, having risen from meditation in the afternoon, approached the Buddha, paid respect with his head at the Buddha's feet, and withdrew to sit at one side. He completely told the Buddha about the above matter, [saying]: “Blessed One, having proceeded to speak in this way, did I not get into misrepresenting the Blessed One? On being closely interrogated by others, will I not be censured and fall into an occasion of being defeated? Did I not speak as I should speak? Did I not speak according to the Dharma, did I not speak the Dharma in accordance with the Dharma?”
The Buddha said to Rādha: “You spoke truthfully. You did not misrepresent the Tathāgata. On being closely interrogated by others, you will not be censured and fall into an occasion of being defeated. You spoke as you should speak. You spoke according to the Dharma, speaking the Dharma in accordance with the Dharma. Why is that? Rādha, bodily form is dukkha. One goes forth under the Tathāgata to cultivate the holy life for the sake of understanding that dukkha. Feeling … perception … formations … consciousness is dukkha. One goes forth under the Tathāgata to cultivate the holy life for the sake of understanding that dukkha.”
When the Buddha had spoken this discourse, hearing what the Buddha had said the monk Rādha was delighted and received it respectfully.