Saṃyukta Āgama 620 The Monkey
Thus have I heard. At one time, the Buddha was in Rājagṛha, in the Kalandaka Bamboo Garden. At that time, the Bhagavān said to the bhikṣus, “In the Himalayas, in some cold and precipitous places, there may be no monkeys and yet there may be people. There are also mountainous places where monkeys reside but where there are no people. There are also mountainous places where people and animals reside together.
“In a place traveled by monkeys, a hunter smears a sticky adhesive on the grass. There are smart monkeys who will avoid it and leave. However, the foolish monkey cannot avoid it. With its hand it gives a little touch, and then its hand is stuck. Then with its second hand it tries to free itself, but its second hand becomes stuck. With its feet it tries to free itself, but then its feet become stuck. With its mouth it tries to gnaw at the grass, and its mouth too becomes stuck. Trapped in five places, lying upon the ground, the hunter arrives and promptly spears it, shoulders it, and departs.
“You bhikṣus should know: that foolish monkey strayed from the realm of its father and mother. Wandering into other realms caused this suffering and affliction. Just so is the bhikṣu who stays in a village like a foolish person. In the morning, he puts on his robes and grabs his bowl, and enters the city to beg for alms. Unskillful in protecting his body, he does not guard the doors of his sense faculties. His eyes see forms and he gives rise to defiling attachment. His ears hear, nose smells, tongue tastes, and body feels, all giving rise to defiling attachment. The inner faculty of the foolish bhikṣu has been bound to external objects with five cords, in the will of Māra.
“For this reason, bhikṣus, you should train thusly: staying in the realm of your father and mother, and abiding there. Do not walk into other places and other realms. How does a bhiksu dwell in the place of the realm of his father and mother? That is to say in the Four Bases of Mindfulness:  abiding in mindfulness of the body, observing the body;  of sensation;  of the mind; and  abiding in mindfulness of dharmas, observing dharmas.”
After the Buddha had spoken this sūtra, the bhikṣus heard what the Buddha had said, and joyfully practiced in accordance.