The Buddha teaches Rāhula about the importance of truthfulness, teaching him to not tell an intentional lie even as a joke.
Thus have I heard - At one time, the Blessed One was dwelling in Rājagaha, in the Bamboo Grove, the Squirrel Sanctuary.
Now at that time, the venerable Rāhula was dwelling at Ambalaṭṭhika. Then the Blessed One, having arisen from seclusion in the evening time, approached Ambalaṭṭhika, where the venerable Rāhula was. The venerable Rāhula saw the Blessed One approaching from afar. Having seen him, he prepared a seat and water for his feet. The Blessed One sat down on the prepared seat. Having sat down, he washed his feet. The venerable Rāhula also, having paid homage to the Blessed One, sat down to one side.
An Intentional Lie
Then the Blessed One placed a small amount of leftover water in the water vessel and addressed the venerable Rāhula: "Rāhula, do you see this small amount of leftover water in the water vessel?"
"Yes, venerable sir."
"Even so little, Rāhula, is the asceticism of those who do not feel shame in telling an intentional lie."
Then the Blessed One, having discarded the small amount of leftover water, addressed the venerable Rāhula: "Do you see this small amount of leftover water discarded, Rāhula?"
"Yes, venerable sir."
"Thus, discarded, Rāhula, is the asceticism of those who do not feel shame in telling an intentional lie."
Then the Blessed One, having overturned that water vessel, addressed the venerable Rāhula: "Do you see this overturned water vessel, Rāhula?"
"Yes, venerable sir."
"Thus overturned, Rāhula, is the asceticism of those who do not feel shame in telling an intentional lie."
Then the Blessed One, having turned the water vessel upright, addressed the venerable Rāhula: "Rāhula, do you see this hollow and empty water vessel?"
"Yes, venerable sir."
"Even so hollow and empty, Rāhula, is the asceticism of those who do not feel shame in telling an intentional lie."
Simile Of The King's Elephant
"Just as, Rāhula, a king's elephant with sharp tusks, of a mighty stature, well bred, accustomed to the battlefield, engaged in battle, acts with his front feet, acts with his hind feet, acts with his front body, acts with his hind body, acts with his head, acts with his ears, acts with his tusks, acts with his tail; however, he protects his trunk. Then his rider would think: 'This king's elephant with sharp tusks, mighty stature, well bred, accustomed to the battlefield, engaged in battle, acts with his front feet, acts with his hind feet, and acts with his tail; however, it protects his trunk. He has not yet given up his life.'
When, Rāhula, a king's elephant with sharp tusks, mighty stature, well bred, accustomed to the battlefield, engaged in battle, acts with his front feet, acts with his hind feet, acts with his front body, acts with his hind body, acts with his head, acts with his ears, acts with his tusks, acts with his tail, and acts with his trunk, then his rider would think: 'This king's elephant with sharp tusks, mighty stature, well bred, accustomed to the battlefield, engaged in battle, acts with his front feet, acts with his hind feet, acts with his front body, acts with his hind body, acts with his head, acts with his ears, acts with his tusks, acts with his tail, and acts with his trunk. He has given up his life. Now there is nothing that the king's elephant will not do.'
Just so, Rāhula, for anyone who does not feel shame in telling an intentional lie, there is no evil that they will not do, I say. Therefore, Rāhula, you should train yourself thus: 'I will not tell an intentional lie even as a joke.' This is how you should train yourself, Rāhula.
Related Teachings:
Five factors of well-spoken speech (AN 5.198) - Speech endowed with these five factors is well-spoken, not ill-spoken, blameless, and irreproachable to the wise.
A line drawn in water | A teaching on speech (AN 3.132) - The Buddha is sharing a guidance here on how to harmoniously interact with others, even in the face of hostility. This is a quality one will gradually cultivate as they work towards enlightenment.
Who to not associate with (AN 3.27) - The Buddha shares guideline on choosing one's associations and company. For it is through associations that one can decline, avoid decline or grow in qualities.
