Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4.6

4.1.4.6

tattadavakLptameva | yadbrāhmaṇo 'rājanyaḥ syādyadyu rājānaṃ labheta samṛddhaṃ tadetaddha tvevānavakLptaṃ yatkṣatriyo 'brāhmaṇo bhavati yaddha kiṃ ca karma kurute prasūtam brahmaṇā mitreṇa na haivāsmai tatsamṛdhyate tasmādu kṣatriyeṇa karma kariṣyamāṇenopasartavya eva brāhmaṇaḥ saṃ haivāsmai tadbrahmaprasūtaṃ karmardhyate

Hence it is quite proper that a Brâhman should be without a king, but were he to obtain a king, it would be conducive to the success (of both). It is, however, quite improper that a king should be without a Brâhman, for whatever deed he does, unsped by Mitra, the priesthood, therein he succeeds not. Wherefore a Kshatriya who intends to do a deed ought by all means to resort to a Brâhman, for he verily succeeds only in the deed sped by the Brâhman.