At Sāvatthī.

“What do you think, Rāhula? Are sights permanent or impermanent?”

“Impermanent, sir.” … “… sounds … smells … tastes … touches … Are thoughts permanent or impermanent?”

“Impermanent, sir.” …

“Seeing this, a learned noble disciple grows disillusioned with sights, sounds, smells, tastes, touches, and thoughts. Being disillusioned, desire fades away. …”