Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.37

3.2.1.37

atha yaddīkṣitaḥ | ṛcaṃ vā yajurvā sāma vābhivyāharatyabhisthiramabhisthiramevaitadyajñamārabhate tasmādamutraivāṅgulīrnyacedamutra vācaṃ yacet

And when the consecrated (after restraining his speech) utters either a rik, or a sâman, or a yagus, he thereby takes a firmer and firmer hold of the sacrifice: let him therefore turn in his fingers and restrain his speech on that (former) occasion.