The Sutra of the Great Treasure Accumulation, Spoken by the Buddha to Mahakasyapa, Volume 4
The Buddha said to Kasyapa, "Consider a person skilled in taming horses. There is a horse of wild temperament, difficult to subdue. This person, through training, naturally makes it gentle and good. Kasyapa! In the same way, a monk who is in accord with the Dharma and upholds the precepts may have a mind and consciousness that are restless and difficult to control. Yet, through the discipline and restraint of this monk, they are subdued, freed from anger and other defilements, and become unshakable and still. I now speak this verse:
"Just as a wild and unruly horse, when met by a skilled trainer,
is gradually tamed and subdued, soon becoming gentle and obedient,
so too a monk who practices diligently, upholding the precepts with care,
tames and calms the restless mind, bringing it to a state of peace and purity."
The Buddha said to Kasyapa, "Suppose a person has a tumor in their throat, which destroys their vital energy and causes them great suffering. Kasyapa, in the same way, if someone clings deeply to the idea of 'self,' they will bring immense suffering upon themselves in the future. On this, I now speak these verses:"