Srimad Bhagavatam Canto 11, Chapter 17, Text 40

SB 11.17.40

 sarvesam ca dvi-janmanam
pratigraho ’dhyapanam ca
 brahmanasyaiva yajanam
All twice-born men — brahmanas, ksatriyas and vaisyas — must perform sacrifice, study the Vedic literature and give charity. Only the brahmanas, however, accept charity, teach the Vedic knowledge and perform sacrifice on behalf of others.
All civilized men must participate in sacrificial performances, give charity and study the Vedic literature. The best of the twice-born, namely the brahmanas, are specifically empowered to lead sacrificial performances on behalf of all members of society, teach everyone Vedic knowledge and receive everyone’s charity. Without the assistance or participation of qualified brahmanas, the lower classes cannot properly study the Vedic literature, perform sacrifices or give in charity, because they do not have the required intelligence to perfectly execute such functions. When ksatriyas and vaisyas take shelter of bona fide brahmanas, they are able to properly execute their respective duties, and society functions smoothly and efficiently.
Srimad Bhagavatam Canto 11, Chapter 17, Text 39
Srimad Bhagavatam Canto 11, Chapter 17, Text 41