Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.4.2

4.3.4.2

sa eṣa yajño hato na dadakṣe | taṃ devā dakṣiṇābhiradakṣayaṃstadyadenaṃ dakṣiṇābhiradakṣayaṃstasmāddakṣiṇā nāma tadyadevātra yajñasya hatasya vyathate tadevāsyaitaddakṣiṇābhirdakṣayatyatha samṛddha eva yajño bhavati tasmāddakṣiṇā dadāti

When slain, that sacrifice was no longer vigorous. By means of dakshinâs (gifts to the priests) the gods invigorated it: hence the name dakshinâ, because thereby they invigorated (dakshay) it. Whatever, therefore, fails in this sacrifice, when slain, that he now invigorates by means of gifts to the priests; then the sacrifice becomes indeed successful: for this reason he makes gifts to the priests.