Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.3.27

2.2.3.27

sa vai varṣāsvādadhīta | varṣā vai sarva ṛtavo varṣā hi vai sarva ṛtavo 'thādo varṣamakurmādo varṣamakurmeti saṃvatsarāntsampaśyanti varṣā ha tveva sarveṣāmṛtūnāṃ rūpamuta hi tadvarṣāsu bhavati yadāhurgrīṣma iva vā adyetyuto tadvarṣāsu bhavati yadāhuḥ śiśira iva vā adyeti varṣādidvarṣāḥ

There are either twelve or thirteen syllables (in these six case-forms). Now there are either twelve or thirteen months in a year; and the year, the seasons, he (Agni) entered: out of the seasons he accordingly thereby produces him. In order to avoid sameness, no two (of these forms) are alike; but (the fault of) sameness he would undoubtedly commit, were any two of them alike. The characteristic form of the fore-offerings is (alternately), 'May they accept,' 'May it (or he) accept;' and that of the after-offerings is, 'For the abundant obtainment of abundant gift.'