Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.45

14.2.2.45

taṃ yatprathamaya]ñe pravṛñjyāt eṣo'sya taptaḥ śuśucānaḥ prajāṃ ca paśūṃśca pradahedatho āyuḥ pramāyuko yajamānaḥ syāttasmāddvitīye vaiva tṛtīye vā

And were he to perform the Pravargya at the first Soma-sacrifice, that (Mahâvîra) of his, when heated and ablaze, would burn up his family and cattle, and also his life, and the Sacrificer would be liable to perish: therefore (let him perform it) at the second or third (sacrifice).