There are three branches of the law. Sacrifice, study, and charity are the first. - Khandogya Upanishad, 2nd Prapathaka, 23rd Khanda: 1

Penance, liberality, righteousness, kindness, truthfulness, these form his Dakshinas. - Khandogya Upanishad, 3rd Prapathaka, 17th Khanda: 4

There lived once upon a time Ganasruti Pautrayana (the great-grandson of Ganasruta), who was a pious giver, bestowing much wealth upon the people, and always keeping open house. He built places of refuge everywhere, wishing that people should everywhere should eat of his food. Once in the night some Hamsas (flamingoes) flew over his house, and one flamingo said to another: 'Hey, Bhallaksha Bhallaksha (short-sighted friend). The light (glory) of Ganasruti Pautrayana has spread like the sky. Do not go near, that it may not burn thee.' - Khandogya Upanishad, 4th Prapathaka, 1st Khanda: 1-2

But they who living in a village practice (a life of) sacrifices, works of public utility, and alms, they go to the smoke, from smoke to night, from night to the dark half of the moon, from the dark half of the moon to the six months when the sun goes to the south. But they do not reach the year. From the months they go to the world of the fathers, from the world of the fathers to the ether, and from the ether to the moon. That is Soma, the king. Here they are loved (eaten) by the Devas, yes, the Devas love (eat) them. - Khandogya Upanishad, 5th Prapathaka, 10th Khanda: 3-4

Those whose conduct has been good, will quickly attain some good birth, the birth of a Brahmana, or a Kshatriya, or a Vaisya. But those whose conduct has been evil, will quickly attain an evil birth, the birth of a dog, or a hog, or a Kandala. - Khandogya Upanishad, 5th Prapathaka, 10th Khanda: 7

The eighty verses (of the hymn) are alike food with reference to the gods as well as with reference to man. For all these beings breathe and live by means of food indeed. By food (given in alms, etc.) he conquers this world, by food (given in sacrifice) he conquers the other. Therefore the eighty verses (of the hymn) are alike food, with reference to the gods as well as with reference to man. - Aitareya-Aranyaka, 2nd Aranyaka, 1st Adhyaya, 2nd Khanda: 13

Now to that prana, which is Brahman, all these deities (mind, speech, eye, ear) bring an offering, though he asks not for it, and thus to him who knows this all creatures bring an offering, though he asks not for it. For him who knows this, there is this Upanishad (secret vow), "Beg not!" As a man who has begged through a village and got nothing sits down and says, "I shall never eat anything given by those people," and as then those who formerly refused him press him (to accept their alms), thus is the rule for him who begs not, but the charitable will press him and say, "Let us give to thee." - Kaushitaki Upanishad, 2nd Adhyaya: 1

He who has three times performed this Nakiketa rite, and has been united with the three (father, mother, and teacher), and has performed the three duties (study, sacrifice, almsgiving) overcomes birth and death. When he has learnt and understood this fire, which knows (or makes us know) all that is borh of Brahman, which is venerable and divine, then he obtains everlasting peace. - Katha Upanishad, 1st Adhyaya, 1st Valli: 17

Death said: "The good is one thing, the pleasant another; these two, having different objects, chain a man. It is well with him who clings to the good; he who chooses the pleasant, misses his end. The good and the pleasant approach man: the wise goes round about them and distinguishes them. Yea, the wise prefers the good to the pleasant, but the fool chooses the pleasant through greed and avarice. Thou, O Nakiketas, after pondering all pleasures that are or seem delightful, hast dismissed them all. Thou has not gone into the road that leadeth to wealth, in which many men perish. Wide apart and leading to different points are these two, ignorance and what is known as wisdom. - Katha Upanishad, 1st Adhyaya, 2nd Valli: 1-4

Let him never turn away ( a stranger) from his house, that is the rule. Therefore a man should by all means acquire much food, for (good) people say (to the stranger): "There is food ready for him." If he gives food amply, food is given to him amply. If he gives food fairly, food is given to him fairly. If he gives food meanly, food is given to him meanly. - Taittiriyaka Upanishad, 3rd Valli, 10th Anuvaka: 1

That Self is indeed Brahman, consisting of knowledge, mind, life, sight, hearing, earth, water, wind, ether, light and no light, desire and no desire, anger and no anger, right or wrong, and all things. Now as a man is like this or like that, according as he acts and according as he behaves, so will he be: a man of good acts will become good, a man of bad acts, bad. He becomes pure by pure deeds, bad by bad deeds. And here they say that a person consists of desires. And as is his desire, so is his will; and as is his will, so is his deed; and whatever deed he does, that will be reap. - Brihadaranyaka Upanishad, 4th Adhyaya, 4th Brahmana: 5

The threefold descendants of Pragapati, gods, men, and Asuras (evil spirits), dwelt as Brahmakarins (students) with their father Pragapati. Having finished their studentship the gods said: "Tell us (something), Sir." He told them the syllable Da. Then he said: "Did you understand?" They said: "We did understand. You told us 'Damyata,' Be subdued." "Yes," he said, "you have understood."

Then the men said to him: "Tell us something, Sir." He told them the same syllable Da. Then he said: "Did you understand?" They said: "We did understand. You told us, 'Datta,' Give." "Yes," he said, "you have understood."

The Asuras said to him: "Tell us something, Sir." He told them the same syllable Da. Then he said: "Did you understand?" They said: "We did understand. You told us, 'Dayadham,' Be merciful." "Yes," he said, "you have understood."

The divine voice of thunder repeats the same Da Da Da, that is, Be subdued, Give, Be merciful. Therefore let that triad be taught, Subduing, Giving, and Mercy. - Brihadaranyaka Upanishad, 5th Adhyaya, 2nd Brahmana: 1-3