Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.2.18

1.3.2.18

sa etena yajuṣā | sakṛjjuhvāṃ gṛhṇāti tristūṣṇīmetenaiva yajuṣā sakṛdupabhṛti gṛhṇāti sapta kṛtvastūṣṇīmetenaiva yajuṣā sakṛddhruvāyāṃ gṛhṇāti tristūsṇīṃ tadāhustristrireva yajuṣā gṛhṇīyātrivṛddhi yajña iti tadu nu sakṛtsakṛdevātro hyeva trirgṛhītaṃ sampadyate

Once he puts (butter with the sruva) into the guhû with this formula, three times silently. With the same formula he puts (butter) once into the upabhrit, seven times silently. With the same formula he puts once (butter) into the dhruvâ, three times silently. Now, as to this, they say, 'Thrice he should take with the formula in each case, for threefold is the sacrifice.' Nevertheless (it is done) only once with each (spoon), for it is just in this way that the taking thrice (with a formula) is accomplished.