Śatapatha Brāhmaṇa ("of a hundred paths") 6.8.2.5

6.8.2.5

tribhirabhyavaharati | trivṛdagniryāvānagniryāvatyasya mātrā tāvataivainadetadabhyavaharatyekenāgre 'tha dvābhyāṃ dvābhyāṃ vāgre 'thaikena dvistu kṛtvo 'bhyavaharati tadye dvipādāḥ paśavastairevainadetadabhyavaharati

With three (verses) he throws (the ashes into the water),--threefold is Agni: as great as Agni is, as great as is his measure, by so much he thus throws them down. First with one (prayer), and then with two; or first with two, and then with one,--but at two separate times he throws them down: he thus throws them down by means of the two-footed animals.