Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.36

3.2.1.36

atra haike | aṅguliśca nyacanti vācaṃ ca yacantyato hi kiṃ ca na japiṣyanbhavatīti vadantastadu tathā na kuryādyathā parāñcaṃ dhāvantamanulipseta taṃ nānulabhetaivaṃ ha sa yajñaṃ nānulabhate tasmādamutraivāṅgulīrnyacedamutra vācaṃ yacet

It is only now that some bend the fingers inward and restrain their speech, because, they argue, only from now will he not have to mutter anything. But let him not do so; for in like manner as if one were to try to overtake some one who runs away, but could not overtake him, so does he not overtake the sacrifice. Let him therefore turn in his fingers and restrain his speech on that (former) occasion.