From Plutarch’s life of Numa (ch. 15, pg. 358 of the Loeb vol. 1 of Lives) and below another version from Ovid’s Fasti (3.330, also in the Loeb edition). Found via a side remark in George Dumezil’s Mitra-Varuna.
But nothing can be so strange as what is told about [Numa’s] conversation with Jupiter … Some, however, say that it was not the imps themselves who imparted the charm [of onions, hair, and little fish against thunder and lightning], but that they called Jupiter down from heaven by their magic, and that this deity angrily told Numa that he must charm thunder and lightning with “heads.” “Of onions?” asked Numa, filling out the phrase. “Of men,” said Jupiter. Thereupon Numa, trying once more to avert the horror of the prescription, asked, “with hair?” “Nay,” answered Jupiter, “with living—” “fish?” added Numa, as he had been taught by Egeria to say. Then the god returned to heaven in a gracious mood,—“hileos,” as the Greeks say,—and the place was called Ilicium from this circumstance; and that is the way the charm was perfected.
πᾶσαν δὲ ὑπερβέβληκεν ἀτοπίαν τὸ ὑπὲρ τῆς τοῦ Διὸς ὁμιλίας ἱστορούμενον. μυθολογοῦσι γὰρ εἰς τὸν Ἀβεντῖνον λόφον οὔπω μέρος ὄντα τῆς πόλεως οὐδὲ συνοικούμενον, ἀλλ᾿ ἔχοντα πηγάς τε δαψιλεῖς ἐν αὑτῷ καὶ νάπας σκιεράς, φοιτᾶν δύο δαίμονας, Πῖκον καὶ Φαῦνον· οὓς τὰ μὲν ἄλλα Σατύρων ἄν τις ἢ Πανῶν γένει προσεικάσειε, δυνάμει δὲ φαρμάκων καὶ δεινότητι τῆς περὶ τὰ θεῖα γοητείας λέγονται ταὐτὰ τοῖς … ἔνιοι δὲ οὐ τοὺς δαίμονάς φασιν ὑποθέσθαι τὸν καθαρμόν, ἀλλ᾿ ἐκείνους μὲν καταγαγεῖν τὸν Δία μαγεύσαντας, τὸν δὲ θεὸν ὀργιζόμενον τῷ Νομᾷ προστάσσειν ὡς χρὴ γενέσθαι τὸν καθαρμὸν κεφαλαῖς· ὑπολαβόντος δὲ τοῦ Νομᾶ, “κρομμύων;” εἰπεῖν, “ἀνθρώπων·” τὸν δὲ αὖθις ἐκτρέποντα τὸ τοῦ προστάγματος δεινὸν ἐπερέσθαι, “θριξίν;” ἀποκριναμένου δὲ τοῦ Διός, “ἐμψύχοις,” ἐπαγαγεῖν τὸν Νομᾶν, “μαινίσι;” ταῦτα λέγειν ὑπὸ τῆς Ἠγερίας δεδιδαγμένον. καὶ τὸν μὲν θεὸν ἀπελθεῖν ἵλεω γενόμενον, τὸν δὲ τόπον Ἰλίκιον ἀπ᾿ ἐκείνου προσαγορευθῆναι καὶ τὸν καθαρμὸν οὕτω συντελεῖσθαι.
Sure it is the tops of the Aventine trees did quiver, and the earth sank down under the weight of Jupiter. The king’s heart throbbed, the blood shrank from his whole body, and his bristling hair stood stiff. When he came to himself, “King and father of the high gods,” he said, “vouchsafe expiations sure for thunderbolts, if with pure hands we have touched thine offerings, and if for that which now we ask a pious tongue doth pray.” The god granted his prayer, but hid the truth in sayings dark and tortuous, and alarmed the man by an ambiguous utterance. “Cut off the head,” said he.a The king answered him, “We will obey. We’ll cut an onion, dug up in my garden.” The god added, “A man’s.” “Thou shalt get,” said the other, “his hair.” The god demanded a life, and Numa answered him, “A fish’s life.” The god laughed and said, “See to it that by these things thou dost expiate my bolts, Ο man whom none may keep from converse with the gods!
constat Aventinae tremuisse cacumina silvae,
terraque subsedit pondere pressa Iovis.
corda micant regis, totoque e corpore sanguis
fugit, et hirsutae deriguere comae.
ut rediit animus, “da certa piamina” dixit
“fulminis, altorum rexque paterque deum,
si tua contigimus manibus donaria puris,
hoc quoque, quod petitur, si pia lingua rogat.”
adnuit oranti, sed verum ambage remota
abdidit et dubio terruit ore virum.
“caede caput” dixit: cui rex “parebimus,” inquit
“caedenda est hortis eruta cepa meis.”
addidit hic “hominis”: “sumes” ait ille “capillos.”
postulat hic animam, cui Numa “piscis” ait.
risit et “his” inquit “facito mea tela procures,
o vir conloquio non abigende deum.