Kummopama Sutta [SN 35.240, PTS 4.178–4.179]

3.

The Discourse on the Simile of the Tortoise

240. "Once in the past, monks, a tortoise was engaged in foraging along the riverbank in the evening. A jackal too, monks, was engaged in foraging along the riverbank in the evening. The tortoise, monks, saw the jackal from afar engaged in foraging. Having seen him, having drawn in its limbs with the snout as the fifth into its own shell, it remained inactive, silent, and kept still. The jackal too, monks, saw the tortoise from afar engaged in foraging. Having seen it, he approached the tortoise; having approached, he stood waiting by the tortoise - 'When this tortoise extends one or another of its limbs with the snout as the fifth, right there having seized it, having torn it out, I will eat it.' When, monks, the tortoise did not extend one or another of its limbs with the snout as the fifth, then the jackal, disenchanted with the tortoise, departed, not obtaining a chance.

"So too, monks, Māra the Evil One is constantly and continuously waiting for you too - 'Perhaps I might obtain a chance through their eye, etc. or I might obtain a chance through their tongue, etc. or I might obtain a chance through their mind.' Therefore, monks, dwell with guarded doors in the sense faculties. Having seen a visible form with the eye, do not be ones who grasp at signs, nor ones who grasp at features. Since, if you were to dwell with the eye faculty unrestrained, covetousness, displeasure, and evil unwholesome mental states would flow in upon you, proceed to restrain it, guard the eye faculty, commit to restraint of the eye faculty. Having heard a sound with the ear... Having smelled an odour with the nose. Having tasted a flavour with the tongue. having touched a tangible object with the body... Having cognised a mental phenomenon with the mind, do not be ones who grasp at signs, nor ones who grasp at features. Since, if you were to dwell with the mind faculty unrestrained, covetousness, displeasure, and evil unwholesome mental states would flow in upon you, proceed to restrain it, guard the mind faculty, commit to restraint of the mind faculty. When you, monks, dwell with guarded doors in the sense faculties, then Māra the Evil One, disenchanted with you too, will depart, not obtaining a chance - like the jackal from the tortoise."