​MN 61 Ambalatthikarahulovada Sutta: Advice to Rahula at Ambalatthika 

​Thus have i heard. 

On one occasion the Blessed One was living at Rajagaha in the Bamboo Grove, the Squirrels' Sanctuary. Now on that occasion the venerable Rahula was living at Ambalatthika. Then in the evening, the Blessed One rose from meditation and went to the venerable Rahula at Ambalatthika. Venerable Rahula saw the Blessed One coming in the distance and made a seat ready and set out water for washing the feet. The Blessed One sat down on the seat made ready and washed his feet. The venerable Rahula paid homage to him and sat down at one side. Then the Blessed One left a little water in the water vessel and asked the venerable Rahula: "Rahula, do you see this little water left in the water vessel?"
Rahula: "Yes, venerable sir."
Buddha: "Even so little, Rahula, is the recluse-ship of those who are not ashamed to tell a deliberate lie."

Then the Blessed One threw away the little water that was left and asked the venerable Rahula: "Rahula, do you see that little water that was thrown away?"
Rahula: "Yes, venerable sir."
Buddha: "Even so, Rahula, those who are not ashamed to tell a deliberate lie have thrown away their recluseship." Then the Blessed One turned the water vessel upside down and asked the venerable Rahula: "Rahula, do you see this water vessel turned upside down?"
Rahula: "Yes, venerable sir."
Buddha: "Even so, Rahula, those who are not ashamed to tell a deliberate lie have turned their recluse-ship upside down."

Then the Blessed One turned the water vessel right way up again and asked the venerable Rahula: "Rahula, do you see this hollow, empty water vessel?"
Rahula: "Yes, venerable sir."
Buddha: "Even so hollow and empty, Rahula, is the recluse-ship of those who are not ashamed to tell a deliberate lie." 
​ "Suppose, Rahula, there were a royal tusker elephant with tusks as long as chariot poles, full-grown in stature, high-bred, and accustomed to battle. In battle he would perform his task with his forefeet and his hindfeet, with his forequarters and his hindquarters, with his head and his ears, with his tusks and his tail, yet he would keep back his trunk. Then his rider would think: 'This royal tusker elephant with tusks as long as chariot poles.. .performs his task in battle with his forefeet and his hindfeet... yet he keeps back his trunk. He has not yet given up his life.' But when the royal tusker elephant...performs his task in battle with his forefeet and his hindfeet, with his forequarters and his hindquarters, with his head and his ears, with his tusks and his tail, and also with his trunk, then his rider would think: 'This royal tusker elephant with tusks as long as chariot poles...performs his task in battle with his forefeet and his hindfeet...and also with his trunk. He has given up his life. Now there is nothing this royal tusker elephant would not do.' "

"So too, Rahula, when one is not ashamed to tell a deliberate lie, there is no evil that one would not do. Therefore, Rahula, you should train thus: 'I will not utter a falsehood even as a joke.'"

"What do you think, Rahula? What is the purpose of a mirror?"
Rahula:"For the purpose of reflection, venerable sir."
Buddha:"So too, Rahula, an action with the body should be done after repeated reflection; an action by speech should be done after repeated reflection; an action by mind should be done after repeated reflection. Rahula, when you wish to do an action with the body, you should reflect upon that same bodily action thus: 'Would this action that I wish to do with the body lead to my own affliction, or to the affliction of others, or to the affliction of both? Is it an unwholesome bodily action with painful consequences, with painful results?' When you reflect, if you know: 'This action that I wish to do with the body would lead to my own affliction, or to the affliction of others, or to the affliction of both; it is an unwholesome bodily action with painful consequences, with painful results,' then you definitely should not do such an action with the body. But when you reflect, if you know: 'This action that I wish to do with the body would not lead to my own affliction, or to the affliction of others, or to the affliction of both; it is a wholesome bodily action with pleasant consequences, with pleasant results then you may do such an action with the body. "
"Also, Rahula, while you are doing an action with the body, you should reflect upon that same bodily action thus: 'Does this action that I am doing with the body lead to my own affliction, or to the affliction of others, or to the affliction of both? Is it an unwholesome bodily action with painful consequences, with painful results?' When you reflect, if you know: 'This action that I am doing with the body leads to my own affliction, or to the affliction of others, or to the affliction of both; it is an unwholesome bodily action with painful consequences, with painful results,' then you should suspend such a bodily action. But when you reflect, if you know: 'This action that I am doing with the body does not lead to my own affliction, or to the affliction of others, or to the affliction of both; it is a wholesome bodily action with pleasant consequences, with pleasant results,' then you may continue in such a bodily action. "

"Also, Rahula, after you have done an action with the body, you should reflect upon that same bodily action thus: 'Does this action that I have done with the body lead to my own affliction, or to the affliction of others, or to the affliction of both? Was it an unwholesome bodily action with painful consequences, with painful results?' When you reflect, if you know: 'This action that I have done with the body leads to my own affliction, or to the affliction of others, or to the affliction of both; it was an unwholesome bodily action with painful consequences, with painful results,' then you should confess such a bodily action to the Teacher or to your wise companions in the holy life. Having confessed it, you should undertake restraint for the future. But when you reflect, if you know: 'This action that I have done with the body does not lead to my own affliction, or to the affliction of others, or to the affliction of both; it was a wholesome bodily action with pleasant consequences, pleasant results,' you can abide happy and glad, training day and night in wholesome states. "

"Rahula, when you wish to do an action by speech...(as above illustration)...you may do such an action by speech if it is wholesome and avoid if it is unwholesome. Also, Rahula, while you are doing an action by speech... (complete as in §10, substituting "speech" for "body")...you may continue in such an action by speech if it is wholesome and avoid if it is unwholesome. Also, Rahula, after you have done an action by speech... (complete as in §11, substituting "speech"for "body")...you can abide happy and glad, training day and night in wholesome states. "
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"Rahula, when you wish to do an action by mind...(complete as in §9, substituting "mind" for "body")...you may do such an action by mind if it is wholesome and avoid if it is unwholesome.  Also, Rahula, while you are doing an action by mind... (complete as in §10, substituting "mind "for "body")...you may continue in such a mental action if it is wholesome and avoid if it is unwholesome . Also, Rahula, after you have done an action by mind... (complete as in §11, substituting "mind "for "body")...you can abide happy and glad, training day and night in wholesome states." 

"Rahula, whatever recluses and brahmins in the past purified their bodily action, their verbal action, and their mental action, all did so by repeatedly reflecting thus. Whatever recluses and brahmins in the future will purify their bodily action, their verbal action, and their mental action, all will do so by repeatedly reflecting thus. Whatever recluses and brahmins in the present are purifying their bodily action, their verbal action, and their mental action, all are doing so by repeatedly reflecting thus. Therefore, Rahula, you should train thus: 'We will purify our bodily action, our verbal action, and our mental action by repeatedly reflecting upon them.'" 

That is what the Blessed One said. The venerable Rahula was satisfied and delighted in the Blessed One's words. 
In summary, the Buddha gave this discourse to his son, the novice monk Rahula who was 7 years old at that time. The Buddha teaches his son the importance of refraining from intentional false speech and the need for reflection prior to, during, and after doing deeds of body, speech, and mind using the simile of water left in the water vessel and reflection from the mirror. Buddha told him that should a body action, speech or mental thought brings harm to self, others or both then one should abstain from doing so. But if a body action, speech or mental thought brings benefit to self, others or both then one should continue to cultivate it. 
​References: 
1. www.accesstoinsight.org
2. https://suttacentral.net/
3. The middle length discourses of the Buddha (Bhikkhu Bodhi)