Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.2.19

1.5.2.19

sa yadi vṛṣṭikāmaḥ syāt | yadīṣṭyā vā yajeta darśapūrṇamāsayorvaiva brūyādvṛṣṭikāmo vā asmīti tatro adhvaryuṃ brūyātpurovātaṃ ca vidyutaṃ ca manasā dhyāyetyabhrāṇi manasā dhyāyetyagnīdhaṃ stanayitnuṃ ca varṣaṃ ca manasā dhyāyeti hotāraṃ sarvāṇyetāni manasā dhyāyeti brahmāṇaṃ varṣati haiva tatra yatraivamṛtvijaḥ saṃvidānā yajñena caranti

Should he (the sacrificer) be desirous of rain, or should he perform a special offering, or even at the new- and full-moon sacrifice itself, he may say, 'Verily, I am desirous of rain!'--and he may also say to the Adhvaryu, 'Ponder thou in thy mind the east wind and the lightning!'--to the Âgnîdhra, 'Ponder thou the clouds in thy mind!'--to the Hotri, 'Ponder thou in thy mind the thunder and rain!'--to the Brahman, 'Ponder thou all, these in thy mind!'--for where the officiating priests thus practice sacrifice with a perfect mutual understanding between them, there it will indeed rain.