SB 11.17.40

 sarveṣāṁ ca dvi-janmanām
pratigraho ’dhyāpanaṁ ca
 brāhmaṇasyaiva yājanam
ijyā — sacrifice; adhyayana — Vedic study; dānāni — charity; sarveṣām — of all; ca — also; dvi-janmanām — those who are twice-born; pratigrahaḥ — acceptance of charity; adhyāpanam — teaching Vedic knowledge; ca — also; brāhmaṇasya — of the brāhmaṇa; eva — only; yājanam — performing sacrifices for others.
All twice-born men — brāhmaṇas, kṣatriyas and vaiśyas — must perform sacrifice, study the Vedic literature and give charity. Only the brāhmaṇas, 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 brāhmaṇas, 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 brāhmaṇas, 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 kṣatriyas and vaiśyas take shelter of bona fide brāhmaṇas, they are able to properly execute their respective duties, and society functions smoothly and efficiently.