This is as I heard. At one time, the Buddha was living in the city of Sāvatthī, in Jeta’s park, at Anāthapiṇḍika’s monastery.
One day, King Pasenadi of the Kosala province went up to the Buddha and exchanged greetings with him. When the greetings and polite conversation were over, he sat down to one side and said to the Buddha, “Does Master Gotama say about himself, ‘I have attained the supreme enlightenment?’”
“If anyone should rightly be said to have attained the supreme enlightenment, it’s me. Great king, I say, ‘I have attained the supreme enlightenment.’”
“Well, there are those teachers who lead a community, and teach a community. They’re well-known and famous religious founders, regarded as holy by many people. That is, Pūraṇa Kassapa, Makkhali Gosāla, Nigaṇṭha Nāṭaputta, Sañjaya Belaṭṭhiputta, Pakudha Kaccāyana, and Ajita Kesakambala. I also asked them whether they claimed to have awakened to the supreme enlightenment, but they made no such claim. So why do you, given that you’re so young in age and recently became a monk?”
“Great king, these four things should not be looked down on or belittled because they are young. What four? A prince or princess in the royal family, a snake, a fire, and a monk. These four things should not be looked down on or belittled because they are young.”
That is what the Buddha said. Then the Blessed One further said:
“A person should not criticize a high-born and famous prince in the royal family, just because he’s young. It’s possible that that prince, some day will become a king, and as the lord of people, he will gain power. And in his anger he’ll order a royal punishment against the person who criticized him. Seeing this danger, one should not belittle a prince and make him angry for the sake of one’s own life.
“If a person sees a small snake in a forest, in a village, or anywhere, he should not look down on it or make fun of it just because it’s little and young. A deadly venomous snake slithers along in the guise of a small snake. That angry snake chases after the foolish man or woman who made fun of it and bites them. Seeing this danger, one should not belittle a snake and make it angry for the sake of one’s own life.
“A fire burns a huge amount of properties and leaves a blackened trail with tiny flames.
A person should not look down and underestimate a tiny flame just because it’s small and young. Once it gets fuel, it’ll become a huge fire. It’ll lash out and burn the foolish man or woman who fueled the fire. Seeing this danger, one should not look down and underestimate a fire for the sake of one’s own life.
“When a forest is burned by fire, it leaves a blackened trail with tiny flames. Those flames burn even huge forests. More flames will come to life with the passing of days and nights.
“Similarly, if someone disrespects a monk of pure virtue, that person will not gain any children or possessions. He will not gain any wealth as inheritance.
He will become childless and heirless. His life will be ruined like a palm tree which is cut down and will never grow again.
Therefore, a wise person, considering his own wellbeing, would always treat these properly: a snake, a fire, a famous prince, and a monk of pure virtue.”
After the Buddha said this, King Pasenadi said to the Buddha, “Excellent, Master Gotama! Excellent! Just as if someone turned up what was upside down, revealed what was hidden, pointed out the path to whoever was lost, or lit a lamp in the dark so people with good eyes could see what’s there, Master Gotama taught me the Dhamma, which is clear in many ways. I go for refuge to Master Gotama, to the Dhamma, and to the Saṅgha. From this day forth, may Master Gotama remember me as a lay follower who has gone for refuge to the Triple Gem for as long as I live.”