Verse 1.8.3.12

sa yadi vṛṣṭikāmaḥ syāt | etenaivādadīta saṃjānāthāṃ dyāvāpṛthivī iti yadā vai dyāvāpṛthivī saṃjānāthe atha varṣati tasmādāha saṃjānāthāṃ dyāvāpṛthivī iti mitrāvaruṇau tvā vṛṣṭyāvatāmiti tadyo varṣasyeṣṭe sa tvā vṛṣṭyāvatvityevaitadāhāyaṃ vai varṣasyeṣṭe yo'yam pavate so'yameka ivaiva pavate so'yam puruṣe'ntaḥ praviṣṭaḥ prāñca pratyañca tāvimau prāṇodānau prāṇodānau vai mitrāvaruṇau tadya eva varṣasyeṣṭe sa tvā vṛṣṭyāvatvityevaitadāha tametenaivādadīta yadā hyeva kadā ca vṛṣṭiḥ samiva tamanaktyāhutimevaitatkarotyāhutirbhūtvā devalokaṃgacāditi

Preview of English translation:

12. Should he desire rain, let him take up (the prastara), with this text (Vajasaneyi Samhita II, 16 d), “Be ye in harmony with each other, O heaven and earth!” for when heaven and earth are in harmony with each other, then indeed it rains: for this reason he says, “be ye in harmony with each other, O heaven and earth!”—“May Mitra and Varuna favour thee with rain!” whereby he says, “may he who rules over the rain favour thee with rain!” Now he that rules over the rain is undoubtedly that blowing one (Vayu, the wind); and he, it is true, blows as one only; but, on entering into man, he becomes a forward and a backward moving one; and they are these two, the out-breathing and the in-breathing. And Mitra and Varuna assuredly are the out-breathing and in-breathing; and hence he says by that (prayer), “may he who rules over the rain favour thee with rain!” Let him then take it up, with this text, for then the rain will at all times be propitious. He anoints it (the prastara): thereby he makes him (the sacrificer) an oblation, thinking, “May he, as an oblation, go to the world of the gods!”

For a detailled translation, including proper diacritics and footnotes, go the full English translation.