Śatapatha Brāhmaṇa ("of a hundred paths") 13.1.2.2

13.1.2.2

tadāhuḥ dvādaśāratnī raśanā kāryā3 trayodaśāratnī3rityṛṣabho vā eṣa ṛtūnāṃ yatsaṃvatsarastasya trayodaśo māso viṣṭapamṛṣabha eṣa yajñānāṃ yadaśvamedho yathā vā ṛṣabhasya viṣṭapamevametasya viṣṭapaṃ trayodaśamaratniṃ raśanāyāmupādadhyāttadyatha ṛṣabhasya viṣṭapaṃ saṃstriyate tādṛktat

Concerning this they say, 'Is the rope to be made twelve cubits long, or thirteen cubits long?' Well, that year is the bull among the seasons, and the thirteenth (or intercalary) month is an excrescence of the year; and this Asvamedha is the bull among sacrifices; and inasmuch as the bull has an excrescence (hump), one may add on a thirteenth cubit to the rope as an excrescence to this (Asvamedha): even as the bull's hump is attached (to his back), suchlike would this be.