Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.3.13

2.4.3.13

tasya prathamajo gaurdakṣiṇā | agryamiva hīdaṃ sa yadījānaḥ syāddarśapūrṇamāsābhyāṃ vā yajetāthaitena yajeta yadyu anījānaḥ syāccātuṣprāśyamevaitamodanamanvāhāryapacane paceyustam brāhmaṇā aśnīyuḥ

The priests' fee for this (sacrifice) consists of the first-born calf (of the season); for that is, as it were, the first-fruits (of the cattle). If he has already performed the new and full-moon offerings, let him first perform those offerings, and thereupon the present (offering of first-fruits). If, on the other hand, he has not yet performed (the new and full-moon offerings), let them cook a kâtushprâsya pap on the southern fire, and let the priests eat it.