Gita Chapter 17.11 to 17.15

Aphalaakaangkshibhiryajno vidhidrishto ya ijyate;
Yashtavyameveti manah samaadhaaya sa saattwikah.
11. That sacrifice which is offered by men without desire for reward as enjoined by the ordinance (scripture), with a firm faith that to do so is a duty, is Sattwic (or pure).

Abhisandhaaya tu phalam dambhaarthamapi chaiva yat;
Ijyate bharatashreshtha tam yajnam viddhi raajasam.
12. The sacrifice which is offered, O Arjuna, seeking a reward and for ostentation, know thou that to be a Rajasic Yajna!

Vidhiheenam asrishtaannam mantraheenam adakshinam;
Shraddhaavirahitam yajnam taamasam parichakshate.
13. They declare that sacrifice to be Tamasic which is contrary to the ordinances of the scriptures, in which no food is distributed, which is devoid of Mantras and gifts, and which is devoid of faith.

Devadwijagurupraajna poojanam shauchamaarjavam;
Brahmacharyamahimsaa cha shaareeram tapa uchyate.
14. Worship of the gods, the twice-born, the teachers and the wise, purity, straightforwardness, celibacy and non-injury—these are called the austerities of the body.

Anudwegakaram vaakyam satyam priyahitam cha yat;
Swaadhyaayaabhyasanam chaiva vaangmayam tapa uchyate.
15. Speech which causes no excitement and is truthful, pleasant and beneficial, the practice of the study of the Vedas, are called austerity of speech.
COMMENTARY: It is said in the Manu Smriti: “One should speak what is true; one should speak what is pleasant; one should not speak what is true if it is not pleasant, nor what is pleasant if it is false. This is the ancient Dharma”. To be an austerity speech should combine all the attributes mentioned in the above verse.

Gita Chapter 3.11 to 3.15

Devaan bhaavayataanena te devaa bhaavayantu vah;
Parasparam bhaavayantah shreyah param avaapsyatha.
11. With this do ye nourish the gods, and may the gods nourish you; thus nourishing one another, ye shall attain to the highest good.

Ishtaan bhogaan hi vo devaa daasyante yajnabhaavitaah;
Tair dattaan apradaayaibhyo yo bhungkte stena eva sah.
12. The gods, nourished by the sacrifice, will give you the desired objects. So, he who enjoys the objects given by the gods without offering (in return) to them, is verily a thief.

Yajnashishtaashinah santo muchyante sarva kilbishaih;
Bhunjate te twagham paapaa ye pachantyaatma kaaranaat.
13. The righteous, who eat of the remnants of the sacrifice, are freed from all sins; but those sinful ones who cook food (only) for their own sake, verily eat sin.

Annaad bhavanti bhootaani parjanyaad anna sambhavah;
Yajnaad bhavati parjanyo yajnah karma samudbhavah.
14. From food come forth beings, and from rain food is produced; from sacrifice arises rain, and sacrifice is born of action.

Karma brahmodbhavam viddhi brahmaakshara samudbhavam;
Tasmaat sarvagatam brahma nityam yajne pratishthitam.
15. Know thou that action comes from Brahma, and Brahma proceeds from the Imperishable. Therefore, the all-pervading (Brahma) ever rests in sacrifice.