Mahabharata 12.72.32

12.72.32

svargaloke ca mahatīṃ śriyaṃ prāpsyasi pāṇḍava |
asaṃbhavaśca dharmāṇāmīdṛśānāmarājasu |
tasmādrājaiva nānyo'sti yo mahatphalamāpnuyāt || 32 ||

[paragraph continues] Do thou, O son of Kunti, observe with great care this duty (of protection). Thou shalt then obtain the reward of righteousness and no grief and pain will be thine. Thou shalt, O son of Pandu, obtain great prosperity in heaven. Merit like this is impossible to be acquired by persons that are not kings. A person, therefore, who is a king, and no other, can succeed in earning such reward of virtue. Possessed of intelligence, thou hast obtained a kingdom. Do thou protect thy subjects righteously. Gratify Indra with offerings of Soma and the friends and well-wishers with the objects of their wishes.'"